JAEA-Review-2010-065.pdf:15.99MB - 日本原子力研究開発機構

4-02
Li Ion Implantation into -rhombohedral Boron:
Carrier Doping for Superconduction
K. Kirihara a) , H. Hyodo b) , T. Nagatochi c) , S. Yamamoto d) , F. Esaka e) ,
H. Yamamoto f) , S. Shamoto f) and K. Kimura c)
a) National Institute of Advanced Industrial Science and Technology, b) Tokyo University of Science,
c) The University of Tokyo, d) Environment and Industrial Materials Research Division, QuBS, JAEA,
e) Division of Environment and Radiation Sciences, NSED, JAEA,
f) Neutron Material Research Center, QuBS, JAEA
Carrier doping into an -rhombohedral boron (-r-B)
crystal is expected to realize superconduction with a higher
1)
transition temperature (Tc) than that of MgB2 . A
twelve-boron-atom (B12) icosahedral cluster is a building
block of -r-B structure (Fig. 1). Theoretical calculation
suggested that high electronic density of states at Fermi
2)
level could be provided by appropriate carrier doping .
Furthermore, high phonon frequency and strong
electron-phonon coupling in boron are important factors for
high Tc. Recently, we observed superconduction in
3)
Li-doped -r-B crystal for the first time . The method of
carrier doping was Li vapor diffusion. However, the
amount of Li in -r-B is still limited because of the
formation of oxide barrier layer or other secondary phases
and therefore Tc is still low (~7 K). Ion implantation is
expected to be one of the effective methods of Li doping for
realizing higher Tc than ever. In boron rich solids, very
little is known about radiation damage by ion implantation.
Only in boron carbide, self recovery of radiation damage in
icosahedral cluster was reported in the study of He-ion
4)
implantation and post annealing . However, radiation
damage in -r-B is not reported. In this study, we report
radiation damage in -r-B by Li-ion implantation. Effect of
carrier doping after post annealing is presented.
Powder sample of -r-B was prepared by annealing of
highly pure (99.99%) amorphous boron at 1,200 o C for 50 h
in vacuum. The powder was formed into pellet by spark
plasma sintering (SPS). Electrical conductivity was
measured for the SPS samples by van der Pauw technique at
2~300 K. Micro-grains (3~5 m in diameter) of a high
purity single crystal were selected for the measurement of
Raman spectroscopy. Implantation of Li + ions with energy
of 150 keV was conducted at ambient temperature. We did
not heat the samples during implantation in order to avoid
vaporization of implanted Li.
According to the Raman spectra of micro-grains, Li
implantation with a fluence of 4.5 × 10 17 ions/cm 2 resulted in
amorphization in the implanted region. Raman spectra of
an -r-B crystal observed after in-situ laser annealing,
indicates recovery of the damage. The temperature of
healing was estimated to be approximately 900 o C.
1 mm)
The ion fluence for SPS samples (3 mm × 3 mm ×
was 1.3 × 10 18 ions/cm 2 . Secondary ion mass spectroscopy
(SIMS) of implanted samples revealed a maximum Li
concentration of 7~8 at% at depth of ~700 nm from the
JAEA-Review 2010-065
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surface. Since the implanted region has significant
radiation damage, the temperature dependence of (plot (a)
in Fig. 2) indicates variable range hopping conduction in
Li-implanted amorphous boron. After rapid annealing of
the implanted sample at 900 o C for 1 min in an Ar
atmosphere, the temperature dependence of (plot (b) in
Fig. 2) was similar to that of the vapor diffusion sample
(nominal composition is Li 1.4B 12). Concentration of Li of
the annealed sample measured by SIMS was 2 at%, in
agreement with that of the vapor diffusion sample estimated
by Rietveld analysis. Since the recovery of radiation
damage could occur after post annealing, carrier doping into
-r-B by Li-ion implantation can be expected, similarly to
the case of vapor diffusion. Additional implantation to
obtain higher Li concentration is in progress.
References
1) K. Soga et al., J. Solid State Chem. 177 (2004) 498.
2) S. Gunji et al., J. Phys. Soc. Jpn. 62 (1993) 2408.
3) T. Nagatochi et al., to be submitted.
4) D. Shimeone et al., J. Nucl. Mater. 277 (2000) 1.
σ (Ω -1 cm -1 )
B 12 icosahedral cluster
Fig. 1 Crystalline structure of -r-B.
10 1
10 0
10 -1
10 -2
10 -3
10 -4
10 -5
10 -6
300 100
T (K)
20 5
(b)
x=0 x=1.0
x=2.5
LixB12
2 3
(10
4
3 /T) 1/4 (K -1/4 )
x=1.4
Fig. 2 Temperature dependence of electrical conductivity
of -r-B. Solid lines represents of the samples
before (a) and after (b) Li ion implantation. Broken
lines represents of the Li vapor diffusion samples.
Number x is the nominal composition of Li.
(a)
3