3FTFBSDI

Comprehensive Study of Sub-grain Boundaries in
Si Bulk Multicrystals(b)Growth direction(a)Position, y /mm

5Over the past decade, Si bulk multicrystal has emerged
as the substrate material for commercial solar cells. To
accelerate the adoption of solar cells to the world, ongoing
research targets further reductions in energy costs byimproving material performance, while reducing manufacturingcosts and raw material costs. We approached this issue fromdeep understanding of crystal physics of Si multicrystals,
namely formation mechanism of multicrystalline structureand generation mechanism and properties of defects. The
former study lead us to proposal of the dendritic casting
method [1]. This article presents the latter study.
Among the defects in the Si bulk multicrystals, we focusedon sub-grain boundaries (sub-GBs) because they are known
to perform as serious carrier recombination sites for solar
cells. To improve the crystal quality of Si multicrystals, wemust comprehensively clarify the physics of sub-GBs, i.e.
1) electrical properties, 2) interaction with impurities andstrain, 3) generation, propagation and vanishing mechanisms
and 4) influence of grain boundary structure and crystal
orientations, as illustrated in Fig.1.
We firstly applied spatially-resolved x-ray diffraction todefects characterization of Si bulk multicrystals to precisely
analyze structure and distribution of sub-GBs. Coordinating
their high angular resolution with crystallographic
multicrystalline structure, we succeeded in detecting the subGBs with small angle difference, as shown in Fig.2. We foundthat sub-GBs distribute locally and tend to propagate to thegrowth direction of the ingot across some grain boundaries.
Following the structural characterization, we revealed theirelectrical properties they are significantly electrically active andwork as current leakage sites for solar cells. This fundamental
CrucibleElectrical properties

Si melt
Directional growthInteraction with impurities
and strain
Generation, propagationand vanishing mechanisms

GBs

Influence of GB structure
and crystal orientation

Fig.1. Research topics of sub-GBs in Si bulk multicrystals.

KINKEN Research Highlights 2008 IMR

4

20

2

3
1

10

0
-1

0

x

0

10mm

10
20
Position, x/mm

-1

y

30

Sub-GB d ensity, d/mm

A new approach, combination of defects characterization
technique using x-ray diffraction and model crystal growthexperiments, was applied to investigate the physics of Si
bulk multicrystals. This provides fundamental knowledge of
sub-grain boundaries, which is of crucial importance toimprove crystal quality of Si bulk multicrystals applicable tohigh efficiency solar cells.

No data

Fig.2. (a) Cross-section of the sample, (b) distribution of subGBs density analyzed by spatially-resolved x-ray diffraction.

study gave common recognition of sub-GBs as dominant
defects of the solar cells.
Although we were able to investigate structure, distribution
and electrical properties of sub-GBs in the practical growthmethod, it is difficult to clarify the generation mechanisms
due to the random and complicated multicrystalline structures.
We therefore developed model crystal growth using multi
seed crystals with controlled configurations, which provide
simplification and systematical control of multicrystalline
structure.
We found that sub-GBs are generated from grainboundaries and their amount depends on the grain boundary
structure. Furthermore, finite element stress analysis
modeling the experimented multicrystalline structure revealed
that local shear stress around the grain boundary is the key
driving force of the sub-GBs generation. This research insist
that controlling the grain boundary structure in the practical
growth method is a reliable way to suppress the sub-GBs
generation.
These fundamental knowledge of sub-GBs should beapplicable to improve the material performance of Si
multicrystals and promise high efficiency solar cells, althoughinteraction with impurities and vanishing mechanism of subGBs remain to be investigated.
References[1] K. Fujiwara, W. Pan, N. Usami, K. Sawada, M. Tokairin,
Y. Nose, A. Nomura, T. Shishido and K. Nakajima, Acta
Mater., 54, 3191 (2006).
[2] N. Usami, K. Kutsukake, K. Fujiwara, and K. Nakajima, J.
Appl. Phys., 102, 103504 (2007).
[3] K. Kutsukake, N. Usami, K. Fujiwara, Y. Nose, and K.
Nakajima, Mater. Trans., 48, 143 (2007).
Contact toKazuo Nakajima (Crystal Physics Division)e-mail: nakasisc@imr.tohoku.ac.jp

Future Energy / Environmental Materials

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3FTFBSDI

Do Si Single Crystal Balls Save the World
from the Global Energy Crisis?
Spherical Si single crystals for solar cell substrates havebeen grown successfully with a yield of almost 100% by
optimizing the supercooling for the recrystallization of Si
droplets that were once fabricated by the jet-dropping
method.

were initially fabricated by jet-dropping method controlling
temperature of the furnace and pressure of atmosphere in
the system.
The as-jetted spherical Si multicrystals were thenannealed at 1000°C for 2-5 hours in a mixed atmosphere ofoxygen and nitrogen to form oxide films (SiO2) on theirsurface. Then the spherical Si multicrystals were melted todroplets on a silica plate in an atmosphere of oxygen, and the
Si droplets were recrystallized to be single crystals at
supercooling of a certain range (Fig. 1). It was found that therecrystallization process occurred unidirectionally in each Si
droplet and recrystallized spherical Si single crystals free of
defects (dislocations and oxidation induced stacking faults,
OSF) was obtained at a supercooling in the range of 12-42°C(Fig. 2). Under this condition, spherical Si single crystals forsolar cell substrates have been grown successfully with ayield of almost 100%. While the crystal growth showed adendritic growth mode and only multicrystals was obtained
when the supercooling was larger than 87°C.

Among solar cells based on various materials, crystalline
Si wafer-based photovoltaic modules, i.e., single- and multicrystalline-Si solar cells, are running ahead, dominating
almost 90% of the world’s photovoltaic (PV) production.
Although Si whose Clarke number is 25.8 is the second mostabundant element in the earth crust, a Si-based solar cell is
not an exception that has to be on the track of the cost
reduction to expand the market besides aiming at the highconversion efficiency. Spherical Si solar cells attract
considerable attention because of the expected feature of
low-waste and low cost fabrication. The advantage of
spherical Si crystals fabricated using dropping method is that
the Si crystals can be fabricated directly from dropping molten
Si without any fabrication process that is always required in
the production of the Si wafer-based devices. This results ina great potential for low-waste and low cost fabrication ofsolar cell substrates.
Increasing the energy conversion efficiency to thetheoretical value has been a super tough challenge to Si
wafer-based PV devices, either single crystalline or
multicrystalline. 1 % increment has required bloody efforts ofa number of people, a couple of decades and plenty ofmoney. Although the conversion efficiency of spherical Sibased solar cells is approximately 2-3 % lower than thatmade from Si wafer-based ones, the increment of theefficiency is remarkable these years. This is because wehave good understanding of degradation of lifetime associatedwith crystalline defects [1,2] and we have also found the
optimum growth condition in terms of supercooling of a Si
droplet for the completely crystalline Si sphere [3].
The recrystallization process of Si droplets was studied.
Several hundreds spherical Si multicrystals per second witha diameter of approximately 400 μm in a teardrop shape

References[1] T. Taishi, T. Hoshikawa, M. Yamatani, K. Shirasawa, X.
Huang, S. Uda and K. Hoshikawa, J. Cryst. Growth, 306, 452(2007).
[2] T. Hoshikawa, T. Taishi, X. Huang, S. Uda, M. Yamatani,
K. Shirasawa and K. Hoshikawa, J. Cryst. Growth, 307, 466(2007).
[3] X. Huang, S. Uda, H. Tanabe, N. Kitahara, H. Arimune
and K. Hoshikawa, J. Cryst. Growth, 307, 341 (2007).
Contact toSatoshi Uda (Crystal Chemistry Division)e-mail: uda@imr.tohoku.ac.jp

Increase of supercoolingSingle crystal growth is possible

Multicrystalline growth

Unidirectional growth

Almost unidirectional growth

Approximate
unidirectional growth

Melt

Melt

Melt

CrystalCrystalCrystal

SilicaSilicaSilicaSilica∆T=25˚C∆T=40˚C∆T=60˚C∆T=90˚C

Dendrite growth
CrystalMelt

Fig. 1 Recrystallization processes in an isolated Si droplet on a silica plate at different
degree of supercooling.

28

Future Energy / Environmental Materials

Fig. 2 Cross-section of the recrystallized
spherical single Si crystal obtained
under the supercooling of 12 ˚C.

IMR KINKEN Research Highlights 2008

3FTFBSDI

Recently, we have reported that the microwave irradiation
process can be applied to the dehydriding (hydrogen release)reaction of LiBH4, a light-weight hydride [1, 2]. The rate of
the temperature increase in LiBH4 by the microwave
irradiation was drastically enhanced above 380 K, which
resulted in a fast dehydriding reaction of LiBH4. At thistemperature, LiBH4 undergoes a structural transition fromorthorhombic (low temperature (LT)) to hexagonal (hightemperature (HT)) phases [3]. From the result of thepermittivity (dielectric constants and loss values)
measurements, LiBH4 is suggested to be an insulator in the
LT phase, while it might be conductive in the HT phase [1].
Therefore, probably due to the conductive loss, the HTphase was rapidly heated. Since hydrogen is covalently
stabilized as [BH4]-, which is then ionically combined with Li+[3], it was predicted that the charge carrier is Li+, and not
H+.
Fig. 1 shows the temperature dependence of theelectrical conductivity of LiBH4 [4]. In the heating process,the electrical conductivity increased with increasing
temperature. A discontinuous jump was observed at around390 K, which corresponds to the structural transition
temperature. The electrical conductivity of the HT phase wasas high as 10-3 Scm-1, which is comparable to those of highLi ionic conductors at room temperature. Below and above
390 K, the temperature dependences of the electricalconductivity showed Arrhenius behaviors. The activationenergies Eaσ for the conduction were evaluated to be of 0.69
eV and 0.53 eV for the LT and HT phases, respectively. In
the cooling process, the discontinuous jump of the electricalconductivity was observed at a lower temperature. The
hysteresis of approximately 10 K was in accordance withthat observed by the differential scanning calorimeter curves,
as shown in the inset of Fig. 1.
The 7Li-NMR data showed a fairly good agreement withthe measured electrical conductivity [4]. Thus, it can beconcluded that the high electrical conductivity of the order of
10-3 Scm-1 in the HT phase originated from the fast Li ionicmotion. Moreover, the calculated electrical conductivity for
dimensional factor n = 4 showed better agreement with themeasured electrical conductivity than that for n = 6.
As shown in Fig. 2, the nearest-neighbor of Li arrange
along the a and b axes without hindrance at the HT phase[4]. The rearrangement might enable the Li+ to migrate alongeach axis.

KINKEN Research Highlights 2008 IMR

10

-2

10

-3

10

-4

10

-5

10

-6

10

-7

423

Temperature (K)373
DSC (a. u.)

323

Endo.

10

-1

-1

The electrical conductivity of lithium borohydride (LiBH4)
jumped by three orders of magnitude due to structuraltransition from orthorhombic to hexagonal phases atapproximately 390 K. The hexagonal phase exhibited a highelectrical conductivity of the order of 10-3 Scm -1. Furthermore,
the conductivity obtained from 7Li-NMR was in good
agreement with the measured electrical conductivity. It was
concluded that the electrical conductivity in the hexagonalphase is due to the Li super-ionic conduction.

Electrical conductivity σ (Scm )Lithium Borohydride: A Light-Weight Hydride Showing“Lithium Super-ionic Conductvity”

360 380 400
Temp. (K)

Heating
Cooling
2.2

2.4

2.6

2.8

-1

3.0

3.2

1000/T (K )Fig. 1 Temperature dependences of the electrical conductivityof LiBH4. Inset shows the differential scanning calorimeter(DSC) of LiBH4. Red and blue lines correspond to data taken
during heating and cooling process, respectively.

Fig. 2 Crystal structures of LiBH4 in the (a) orthorhombic (lowtemperature (LT)) and (b) hexagonal (high temperature (HT))
phases. Blue ball and gray tetrahedron show Li and BH4,
respectively.
References[1] Y. Nakamori, S. Orimo and T. Tsutaoka, Appl. Phys. Lett.,
88, 112104 (2006).
[2] M. Matsuo, Y. Nakamori, K. Yamada and S. Orimo, Appl.
Phys. Lett., 90, 232907 (2007).
[3] S. Orimo, Y. Nakamori, J.R. Eliseo, A. Züttel and C.M.
Jensen, Chem. Rev., 107, 4111 (2007).
[4] M. Matsuo, Y. Nakamori, S. Orimo, H. Maekawa and H.
Takamura, Appl. Phys. Lett., 91, 224103 (2007).
Contact toShin-ichi Orimo (High-Temperature Materials Science Division(Environmental Materials Science))e-mail: orimo@ imr.tohoku.ac.jp

Future Energy / Environmental Materials

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3FTFBSDI

Design of Tetraketone Ligands with Chelate Effectfor Active Materials of Uranium Redox- Flow Battery
Depleted uranium, derived as a major by-product ofenrichment process of the nuclear fuel, has been depositedand its total weight will exceed two million tons in the world by
2015. Since its conventional use in counterweights, shielding
materials, and depleted uranium shell was unsuccessful,
efforts have been paid for potential applications to various
materials. Whereas none of them has been put to practicaluse. Taking account of the electrochemical nature ofuranium, we have proposed uranium redox-flow battery withpotential high energy-efficiency.
We have focused our attention on the inherent nature oflight actinides (An) that there are two couples of isostructuralions, namely Ann+ (n = 3 and 4; n is oxidation number) andAnO2(n-4)+ (n = 5 and 6). Within the couples of the isostructuralions, rapid kinetics of electron transfer (ET) is expected tolower electrochemical overvoltage according to the ButlerVolmer law (Fig. 1). This is clear contrast to the vanadium
redox-flow battery presently in operation consisting of
aqueous electrolyte (V2+/V3+ for negative and VO2+/VO2+ for
positive electrolytes) which suffers from low energy efficiency
due to slow kinetics of the ET especially of VO2+/VO2+ couple
requiring oxygen transfer.
We have demonstrated neptunium (Np) battery withaqueous electrolytes [1, 2]. Our recent study has focused on
the application of uranium (U) as an active material of theredox-flow battery in use of leveling of electric power
generated by natural energy such as wind power.

carbon felt
electrodes

+e
U

4+

-

+

U4+
+
U3+

2+

U

3+

UO 2 + e

UO +2
+
UO 2+
2

UO 2

+

ion exchange
membrane(a)Energy efficiency

1.0
0.9

(i)
(ii)
(iii)

0.8

Np battery
U battery
V battery

0.7
0.6

(iv)

0.5
0

(b)
40
80
120
-2
Current density /mA cm

Fig. 1. (a) Schematic diagram of uranium redox-flow battery. (b)Comparison of energy efficiency of redox-flow batteries using
active materials of neptunium, uranium, and vanadium, andusing that of k0 = 10-1 (i), 10-2 (ii), 10-3 (iii), and 10-4 (iv).

30

Future Energy / Environmental Materials

The β-diketonates of uranium are the first candidates forthe active materials studied. The metal β-diketonates withchemical functionalities and toughness for light and heat
have been applied to electroluminescence (EL) materials
and precursors of metallorganic chemical vapor deposition(MOCVD) for oxide materials.

Fig. 2 Dimerization of ligand yields a dramatic improvementof kinetics and thermodynamics as active materials of
uranium redox-flow battery.

Cyclic voltammetry of β-diketonates of U(VI)/U(V) andU(IV)/U(III) displays multiple couples of peaks and suggests
dissociative ET, i.e. ET followed by chemical reaction (EC),due to smaller effective charge carried by the reduced metal
center. Assuming enhanced stability of the reduced complex,
the electromotive force (EMF) of the single-unit cell of thebattery is estimated more than 1.0 V from the difference
between E1/2 for two-coordinated complexes of U(VI)/U(V)and that for four-coordinated ones of U(IV)/U(III). This value
is comparable to 1.2 V of the vanadium battery. One of thereasonable strategies is use of chelate effect by cyclic or
multidentate ligands such as crown ethers which form stablecomplexes even with alkaline metal ions with least complexing
ability. In order to stabilize the complexes with U(V) andU(III), we have developed dimerized tetraketone ligands (Fig.
2) [3]. By using the tetraketone the demonstration of theuranium battery was carried out by successful charge anddischarge experiments.
References[1] T. Yamamura, N. Watanabe and Y. Shiokawa, J. Alloys
Compds., 408-412C, 1260 (2006).
[2] T. Yamamura, N. Watanabe, T. Yano and Y. Shiokawa, J.
Electrochem. Soc., 152, A830 (2005).
[3] T. Yamamura, K. Shirasaki, H. Sato, Y. Nakamura, H.
Tomiyasu, I. Satoh and Y. Shiokawa, J. Phys. Chem. C, 111,
18812 (2007).
Contact toTomoo Yamamura (Radiochemistry of Metals Division)e-mail: yamamura@imr.tohoku.ac.jp

IMR KINKEN Research Highlights 2008

3FTFBSDI

Spatially-resolved Observation of Glow Discharge Plasma for
Atomic Emission Spectrometry
An imaging spectrograph equipped with a CCD detector
was employed to measure two-dimensional emission images
from a glow discharge plasma in atomic emission spectrometry.
The emission images at Zn I 334.50 nm for a zinc sample
could be obtained. Their emission intensities were not uniform
in the radial direction of the plasma region but became
weaker at larger distance from the central zone. The twodimensional distribution would result from a spatial variationin the excitation efficiency of the plasma and thus provide
useful information for understanding the excitation processes
occurring in the plasma.
A two-dimensional (2D) image of the spectrum emitted
from the glow discharge plasma would give useful information
not only for determining the experimental conditions in theanalytical application but for studying the excitation processes
occurring in the plasma. This paper indicates 2D images ofthe emission signal in the radial direction of the glow discharge
plasma, when a zinc emission line is observed by using a twodimensionally imaging spectrometer having a good spectral
resolution of less than 1 nm.
Experimental
Figure 1 shows the block diagram of the measuringsystem employed. A glow discharge lamp was made in ourlaboratory according to the original model of Grimm, where
the inner diameter of the hollow anode was 8.0 mm and the
distance between the anode and cathode sample wasadjusted to be 0.2-0.4 mm. Emission signals from the glow
discharge lamp, which were observed from the axial directionof the plasma, were conducted through a collimator optics
onto the entrance slit of an image spectrograph (Model 12580,
BunkoKeiki Corp., Japan), and the emission image was thendispersed and detected on a charge-coupled device detector(SensiCam QE Model, PCO Imaging Corp., Germany), where
the 2D image of a particular emission line could be observedin the radial direction of the plasma.[1] A 2D image obtainedby 5 x 5 pixels corresponded to an actual sample area of 0.1
x 0.1 mm2. The spectral resolution was 0.1-1.0 nm dependingon the slit width. Pulsated discharge voltages were applied to
the glow discharge lamp with a high-speed high-voltage
amplifier (HEOPT-B60-L1, Matsusada Precision Inc., Japan),which was controlled with a function generator (Model 4502,
Kikusui Corp., Japan).The discharge parameters aredescribed later. High-purity argon (>99.999 %) was introduced
as the plasma gas after evacuating the chamber to below
10 Pa. The pressure was monitored with a Pirani gauge (GP2, ULVAC Corp., Japan), which had been corrected for pureargon, which were placed between the evacuation port and a

Fig. 2 Two-dimensional emission image of the Zn I 334.50nm line observed from the axial direction of a glow discharge
plasma (a) and the cross-section distribution of the emission
intensity along the radial distance (b).
rotary vacuum pump (GLD-166, ULVAC Engineering Inc.,
Japan). The plasma gas was flowed during the measurementon keeping a chamber pressure of 5.3 x 102 Pa.
Results and Discussion
Figure 2 shows a 2D emission image (a) for the zinc
sample, whose intensities are expressed by mapping withseveral colors, and the cross-section distribution (b) along theradial distance of the plasma.[2] The analytical lines was Zn I
334.50 nm (7.78 eV). For this measurement, a pulsateddischarge mode was employed, where the frequency of thedischarge was 100 Hz, the duty ratio was 10 %, the maximum
voltage was 450 V, and the minimum voltage (offset voltage)
was 50 V. The pulsated discharge is required to accumulate
the emission image in the CCD detector, and it is also useful
to control the sputtering rate appropriately. The slit width of
the spectrometer was 2.0 mm and the gate width of the CCD
detector was 1 ms. The emission signal was stored in the
CCD detector during 32 discharge pulses, and thisaccumulation was 100-times replicated and averaged on the
personal computer. Note that the diameter of the hollow
cathode is 8.0 mm; therefore, this diameter restricts theplasma zone. It can be clearly demonstrated that the emissionof the samples is not irradiated uniformly over the possible
plasma zone but has a large variation in the intensities along
the radial distance. The central portion having a diameter ofabout 2 mm gives the most intense emission. The concentriccircles pattern means that the emission intensity becomes
weaker with increasing the diameter. This effect is probably
because plasma parameters determining the excitation
processes, such as electron density and electron temperature,might be changed in the radial direction of the glow discharge
plasma. We are not surprised at this result, because theplasma region should reach a boundary near the hollow
anode (anode dark space) and the resulting emission is thus
extinguished. However, the shape of the emission zone and theintensity distribution have not been reported in previous papers.
This result could provide novel information for understanding
the excitations occurring in the glow discharge plasma.
References[1] C. Kitaoka and K. Wagatsuma: Anal. Sci., 23, 1261 (2007).
[2] Y. Zenitani and K. Wagatsuma: Anal. Sci., 24, 555 (2008).

Fig. 1 Block diagram of the measuring system.

KINKEN Research Highlights 2008 IMRContacted to
Kazuaki Wagatsuma (Analytical Science Division)e-mail: wagatuma@imr.tohoku.ac.jp

Future Energy / Environmental Materials

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3FTFBSDI

20 T Superconducting Outsert Magnet with a 400 mm Room
Temperature Bore for a 45 T Hybrid Magnet

CuNb/Nb3Sn strand

3+4

4+5

Stainless steel strand

6+1

8+1

700

25

600

20

500
15

400

10

300
200

5
0

Coil hight/2 (mm)The magnetic field is one of the most important
thermodynamical parameters to study physical properties ofthe materials and the physical effects in general. Our aim is
to fabricate a compact and energy-saving laboratory hybrid
magnet with easy operation and maintenance. To succeed itis necessary to develop high-strength Nb3Sn strand cables
with high critical currents and good enough mechanicalproperties to withstand a large electromagnetic force over
500 MPa. We have recently shown that the critical current
greatly enhances when as-reacted Nb3Sn strands arerepeatedly bent at room temperature (prebending treatment)[1]. This critical current enhancement effect of CuNb/Nb3Sn
strands is related to improvement of the upper critical field
and of the transition temperature due to relaxation of thethermal residual strain [2].
In order to investigate the application of the prebending
effect to strand cabling process, two kinds of strand cables
were fabricated by cabling the reacted CuNb/Nb3Sn strands.
Strands and cables were produced by the Furukawa ElectricCompany. We intended to develop the 3+4 strand cableconsisting of three CuNb/Nb3Sn strands and four stainlesssteel strands. The 3 strand cable composed of three CuNb/
Nb3Sn strands was also prepared for comparison. Fig. 1
shows the schematic cross sectional view of the designed
CuNb/Nb3Sn strand cables. To ensure an operation currentof 1000 A for the designed CuNb/Nb3Sn strand cables, thediameter of a CuNb/Nb3Sn strand was determined as afunction of magnetic field. The operation temperature wasselected to be 2.0 K of the superfluid helium condition and itis the temperature of the operation current margin.
The designed outsert will come across the full excitation
current of 1,000 A at 22 T, and the current margin up to 1,000

A will lead to the coil operation stability. The magnetic stored
energy is estimated to be 144 MJ at 20 T. Fig. 2 shows themagnetic field generation at the coil equator. The maximum
hoop stress appears in the outer layer CuNb/Nb3 Sn section
coils, and the hoop stress value of around 480 MPa is
supposed. The maximum compressive stress is calculated to
be 81 MPa for the CuNb/Nb3Sn layer coil and 127 MPa for
the NbTi layer coil. The coil space current density is 22.1 A/
mm2 as low as 62% of the conductor current density for the
innermost layer coil. We found that the 20 T-Ф400 mm RTbore superconducting outsert can be reasonably designed
as compactly as possible.

Magnetic field at the coil equator (T)To construct a high field magnet with compact andenergy-saving design as well as with easy operation andmaintenance, we have been developing high-strength
Nb3Sn strand cables, with maximized superconducting
characteristics and which can withstand a large
electromagnetic force over 500 MPa. A 20 T superconducting
magnet with a room temperature bore (Ф=400 mm) consistingof five layers made of CuNb/Nb3Sn and two layers of NbTi
was designed. The 20 T superconducting coil will be used
as a 20 T outsert to obtain a 45 T hybrid magnet.

100
00 100 200 300 400 500 600 700
r (mm)

Fig. 2. Magnetic fields generated at the coil equator for a
20T-Ф400 mm room temperature bore superconducting
outsert. The layers with blue and green colors are Nb3Sn and
NbTi coils, respectively. The generated magnetic field is 20 T
at the center of the coil. The maximum coil height is 1321 mm
at the L7 NbTi layer coil.

References[1] K. Watanabe, S. Awaji, H. Oguro, G. Nishijima, K. Miyoshi,
and S. Meguro, IEEE Trans. Appl. Supercond., 15, 3564
(2005).
[2] H. Oguro, S. Awaji, G. Nishijima, P. Badica, K. Watanabe,
F. Shikanai, T. Kamiyama, and K. Katagiri, J. Appl. Phys.,101, 103913 (2007).
Contact toKazuo Watanabe (High Field Laboratory for Superconducting
Materials)
e-mail: kwata@imr.tohoku.ac.jp

Fig. 1. Schematic cross section of high strength strand cables
composed of CuNb/Nb3Sn strands and stainless steel
strands.

32

Future Energy / Environmental Materials

IMR KINKEN Research Highlights 2008

3FTFBSDI

Ab initio Study of Single-molecule Rotation Switch Based on
Nonequilibrium Green’s Function Theory
Molecular electronics is expected to be realized after
the ending of silicon technology in 10 years, where the firstprinciples based calculations play important role, since up
to the present there have been difficulty to explore
experimentally the switching properties in nanoscale. The
present work showed an interesting behavior of singlemolecular switch, which is based on the rotation of themolecule. The method used in the paper is based on thenonequilibrium Green’s function and the code has beendeveloped originally in our research group. All the numericalcalculations have been conducted by using the HITACHI
SR11000 supercomputer at the Center for Computational
Materials Science, IMR.
The bistable molecular switches have been studied
theoretically based on the first-principles calculation. The
geometry structures of the switches studied here can betriggered between two symmetrical structures by using an
external applied electric field. I-V characteristic curves of thedifferent molecule configurations have been calculated, anddistinguishability of these characteristic curves indicates aswitching behavior, the performance of which can be
improved significantly by selecting suitable donors andacceptors [1].
Figure 1 shows the presently estimated potential energy
surface of the isolated molecule as a function of dihedral
without bias voltage. The arrows indicate the bistablestructures. We define the left state as 1 and the right one as
0 for one bit of future computing unit. The inset showsgeometries of two states.

would like to see fewer electrons and keep pulling them out,or equivalently, more holes pump into this level from the rightlead. In this case, the energy level acts as a channel, and thecurrent flows through this channel.

Fig. 2. Au-lead/molecule/Au-lead open system (1) and its
energy level diagram (2).
The calculated I-V characteristics for the two structures
are presented in Fig. 3. The current curves of two states canbe distinguished, indicating switching behavior. The current
through state 1 is significantly higher than the current throughstate 0, and the maximum current ratio of the two states isabout 5. This suggests that the switching performance is
acceptable for real applications.

Fig.3. I-V characteristics for donor –OH, and their geometriesof two states and the direction of the applied field are depicted
in inset.
Fig.1. Example of single-molecular rotation switch formolecular device.
Figure 2 gives Au-lead/molecule/Au-lead open system
and its energy level diagram with bias which drives thesystem out of equilibrium. μ1 (μ2) represents the quasi-Fermi
level of the left (right) Au lead. The direction of bias is fromright to left shown in Fig. 2, with (μ1) greater than (μ2). If one
energy level lies between two quasi-Fermi levels (see Fig. 2(2)) , the left lead would like to see more electrons occupying
this level and keep charging them in, while the right one

KINKEN Research Highlights 2008 IMR

Reference[1] Y. Y. Liang, F. Jiang, Y. X. Zhou, H. Chen, R. Note, H.
Mizuseki, and Y. Kawazoe, J. Chem. Phys. 127, 084107(2007).
Contact toYoshiyuki Kawazoe (Materials Design by Computer Simulation
Division)e-mail: kawazoe@imr.edu

Future Energy / Environmental Materials

33