Effect of synthesis temperature on superconducting properties of n-SiC added bulk MgB2 superconductor

Abstract

We study the effect of synthesis temperature on the phase formation in
nano(n)-SiC added bulk MgB2 superconductor. In particular we study: lattice
parameters, amount of carbon (C) substitution, microstructure, critical
temperature (Tc), irreversibility field (Hirr), critical current density (Jc),
upper critical field (Hc2) and flux pinning. Samples of MgB2+(n-SiC)x with
x=0.0, 0.05 & 0.10 were prepared at four different synthesis temperatures i.e.
850, 800, 750, and 700oC with the same heating rate as 10oC/min. We found 750oC
as the optimal synthesis temperature for n-SiC doping in bulk MgB2 in order to
get the best superconducting performance in terms of Jc, Hc2 and Hirr. Carbon
(C) substitution enhances the Hc2 while the low temperature synthesis is
responsible for the improvement in Jc due to the smaller grain size, defects
and nano-inclusion induced by C incorporation into MgB2 matrix, which is
corroborated by elaborative HRTEM (high-resolution transmission electron
microscopy) results. We optimized the the Tc(R=0) of above 15K for the studied
n-SiC doped and 750 0C synthesized MgB2 under 140 KOe field, which is one of
the highest values yet obtained for variously processed and nano-particle added
MgB2 in literature to our knowledge.

Full text

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Effect of synthesis temperature on superconducting properties of n-SiC
added bulk MgB2 superconductor
Arpita Vajpayee1,*, R. Jha1, A. K. Srivastava1, H. Kishan1, M. Tropeano2, C. Ferdeghini2
and V.P.S. Awana1
1Quantum Phenomenon and Applications (QPA) Division, National Physical Laboratory
(CSIR), New Delhi-12, India
2CNR-Spin, corso Perrone 24, 16152 Genova, Italy

We study the effect of synthesis temperature on the phase formation in nano(n)-
SiC added bulk MgB2 superconductor. In particular we study: lattice parameters, amount
of carbon (C) substitution, microstructure, critical temperature (Tc), irreversibility field
(Hirr), critical current density (Jc), upper critical field (Hc2) and flux pinning. Samples of
MgB2+(n-SiC)x with x=0.0, 0.05 & 0.10 were prepared at four different synthesis
temperatures i.e. 850, 800, 750, and 700oC with the same heating rate as 10oC/min. We
found 750oC as the optimal synthesis temperature for n-SiC doping in bulk MgB2 in order
to get the best superconducting performance in terms of Jc, Hc2 and Hirr. Carbon (C)
substitution enhances the Hc2 while the low temperature synthesis is responsible for the
improvement in Jc due to the smaller grain size, defects and nano-inclusion induced by C
incorporation into MgB2 matrix, which is corroborated by elaborative HRTEM (high-
resolution transmission electron microscopy) results. We optimized the the Tc(R=0) of
above 15K for the studied n-SiC doped and 750 0C synthesized MgB2 under 140 KOe
field, which is one of the highest values yet obtained for variously processed and nano-
particle added MgB2 in literature to our knowledge.

Key Words: MgB2 superconductor, nano-particle doping, Flux pinning, Critical current
density
PACS: 74.25.Ha, 74.25.Qt, 74.25.Sv, 74.62.Dh

*Corresponding Author
e-mail-vajpaya@mail.nplindia.ernet.in
Fax No. 0091-11-45609310: Phone No. 0091-11-45609210

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Introduction
The discovery of the superconductivity in MgB2 with Tc = 39 K [1] stimulated
much attention from fundamental to applied research. Due to its relatively high critical
temperature, low cost and transparency of the grain boundaries to the current flow [2],
MgB2 superconductor is one of the best candidates for engineering applications in the
temperature range 20-30 K and can be competitive with traditional Nb based
superconductors and with the so called second generation HTS wires. However, its
critical current density (Jc) is suppressed under magnetic fields due to low upper critical
fields (Hc2) and poor flux pinning in the clean material. Therefore, improvement of Jc in
magnetic field is needed for applications in extensive application fields. There have been
several demonstrations of enhanced vortex pinning and critical current density, Jc, in the
magnesium diboride by partial substitution of boron for carbon, trough doping with SiC,
B4C, carbon nano tube (CNT) and nano-C doping [3-11]. Substitution of C in the B site
also significantly increases the upper critical field, Hc2 [12-14]. However, the resulting
pinning effects by doping were not quantitatively consistent among many research groups
[15]. Since superconducting properties of MgB2 are very sensitive to phase purity, size of
starting boron powder and synthesis conditions so the flux pinning properties of the
samples varied effectively in each paper [16].
Some of us previously found that a significant flux pinning enhancement in MgB2
could be easily achieved using nano SiC as an additive [3]. The Si and C released from
the decomposition of nano SiC at the time of formation of MgB2 formed Mg2Si and
substituted at B sites respectively. The C substitution for B resulted in a large number of
intra-granular dislocations and dispersed nano-size impurities, which are both responsible
for the significant enhancement in flux pinning. However, other micro-structural factors
such as grain size or connectivity of MgB2 grains, which determine the flux pinning
properties of undoped MgB2, have not been well clarified as functions of synthesis
temperature. Based on these backgrounds, here, we study the effect of synthesis
temperature on phase formation, critical temperature (Tc), upper critical field (Hc2),
critical density (Jc), irreversibility field (Hirr) and microstructures of doped MgB2.

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Experimental details
Our polycrystalline MgB2+n-SiCx (x = 0, 5-wt% & 10-wt%) samples were
synthesized by solid-state reaction route. The Mg powder used is from Reidel-de-Haen,
amorphous B powder is from Fluka (of assay 95-97%) and n-SiC powder is from Aldrich
(<100 nm). For synthesis the samples, the stoichiometric amounts of ingredients were
ground thoroughly, palletized using hydraulic press and put in a tubular furnace for 2.5
hours in argon atmosphere at four different temperatures: 700oC, 750oC, 800oC & 850oC.
Finally the furnace cooled in the same atmosphere of argon to room temperature.
Constituent phases of the samples were analyzed by the Rigaku miniflex II powder
x-ray diffractrometer using CuK radiation. The grain morphology and microstructures
were examined by high-resolution transmission electron microscopy (HRTEM) (model
Tecnai G2 F30 STWIN field emission gun supported, operated at the electron accelerating
voltage of 300 kV). The magneto-resistivity was measured with H applied perpendicular
to current direction, using four-probe technique. The magnetization measurements were
carried out on Quantum Design Physical Property Measurement System (PPMS) system.
Critical current density (Jc) was determined from magnetization loop using the extended
Bean model.

Results and Discussion
(A) Phase formations and crystallographic interpretations by powder XRD:
In order to clarify the influence of synthesis temperature (Ts) on the phase
formation of MgB2, X-ray diffraction (XRD) pattern were recorded at room temperature.
Figure 1(a) shows the XRD patterns for all the n-SiC doped samples along with pristine
MgB2 synthesized at 700oC, 750oC, 800oC & 850oC. In case of pure MgB2 all
characteristic peaks are obtained and their respective indexing is shown in the figure
itself. The structure of MgB2 belongs to space group P6/mmm. Powder XRD analysis
revealed that nearly single-phase MgB2 bulks containing only a minor fraction of MgO
(marked by 'o' in figure) were obtained for all the samples heated at above 700oC, while
the samples heated at 700oC showed strong diffraction peaks due to unreacted
magnesium (marked by '$' in the Figure 1(a)) in the sample. The volume fraction of

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unreacted Mg is nearly 25% in pure MgB2 sample synthesized at 700oC. MgO fraction is
almost the same in all the samples irrespective of SiC content and synthesis temperature.
Samples doped with SiC showed a considerable amount of Mg2Si (marked with *
in the figures) and minor quantities of unreacted SiC (+). In particular for both MgB2+n-
SiCx, the {hkl} planes {111}, {200}, {220}, {311} and {400} of the Mg2Si are noticed
clearly in Figure1. At x = 5-wt% doping level the samples consist of a major phase of
MgB2 with minority phase of Mg2Si; increasing the doping level of n-SiC at x = 10-wt%,
the amount of this non-superconducting phase increases. The formation of Mg2Si in SiC-
doped samples indicates the dissociation of the SiC and the reaction of Si with Mg (SiC
starts to react with Mg at a temperature as low as 600oC). In fact, this decomposition of
SiC and the formation of the Mg2Si phase are reported in almost all the SiC doping
studies [3,4,6,17]. No other impurity phases like MgB4, MgB7, Mg2C3 and MgB2C2 are
detected in any one of the samples.
As far as the majority MgB2 phase is concerned, the peak {100} situated between
2 = 33o and 2 = 34o shifts towards the higher angle with increasing doping content of
SiC, indicating the contraction in a-axis in crystal lattice. The lattice parameters, a and c,
of the hexagonal AlB2 type structure of MgB2 are calculated using these peak shifts, and
their variation is reported in Table 1. The change in c with increasing x is relatively small
as compared to a parameter. The a-axis parameter shows a larger drop with x for all the
temperatures. The a decrease indicated the effective partial substitution of B by C [3,4,5-
13,17-18]. The substituent C atoms are readily available from the SiC. The actual C
substitution level for our MgB2 + (SiC)x samples can be estimated indirectly from the
change in lattice parameters using formula x = 7.5 ∆(c/a) where ∆(c/a) is the change in
c/a compared to pure sample [18,19]. Figure 1(b) depicts (from the bottom to the top) the
variation of a-axis lattice parameter, c/a value, actual C substitution level (x) in Mg(B1-
xCx)2 and FWHM of (110) peak with synthesis temperature. Figure 1(b) shows an
increase in c/a value at a particular synthesis temperature as we add the n-SiC in MgB2;
which clearly indicates the presence of the lattice strain in doped samples. The lattice
strain can be attributed to the C substitution in the structure and unreacted C inside the
grains [20]. It is observed that c/a and x values are change moderately for 750, 800 and
850oC samples in comparison to 700oC samples. Except 700oC, the low temperature

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synthesis causes larger values of FWHM, suggesting smaller grain size and imperfect
crystallinity. Figure 1(b) revealed that change in a-axis parameter, c/a values, FWHM
values and actual C substitution level for 700oC samples are lower than those of other
samples, which are heated at relatively higher temperature; hence it seems that 700oC
synthesis temperature is comparatively lower temperature for partial substitution of C at
B site in MgB2 matrix.

(B) DC and AC susceptibility measurements:
Figure 2(a) depicts the DC susceptibility versus temperature (T) plots in FC and
ZFC case for an applied field of 1 mT for the series of samples prepared at 750 oC. It is
evident from the figure that pure MgB2 undergoes a sharp superconducting transition
with an onset at 38.6 K within 1 K temperature interval. All the samples exhibit one-step
transition from normal state to superconducting state, however the transition width
increases a bit by increasing the amount of dopant.
The temperature dependence of the real (Χ’) and the imaginary (Χ’’) parts of the
AC susceptibility is illustrated in Figure 2(b) for all the three samples realized at 750 oC.
For these AC susceptibility measurements, an AC magnetic field of 1 mT rms value with
frequency f = 33 Hz has been used in the absence of a DC field. Below the critical
temperature, a sharp decrease in the real part of the AC susceptibility occurs, which
reflects the diamagnetic shielding. In addition, below Tc a peak appears in Χ’’, reflecting
losses related to the flux penetrating inside the grains. No evidence of a two peaks
behavior (granularity) was detected. As the SiC content increases, the onset of
diamagnetic shielding in Χ’ and the peak position in Χ’’ shift towards lower temperatures.
Figure 3 shows the variation of transition temperature (Tc) with synthesis
temperature (Ts) for the undoped and doped samples. Tc is deduced from the onset
temperature of the diamagnetic transition (Tcdia) in the DC susceptibility measurements.
For the undoped sample Tc shows an increase with increasing preparation temperature,
which can be attributed to the improvement of crystallinity due to higher temperatures.
The Tc for the doped samples depends also on the C substitution level:the competitive
behavior of these two factors produced the curves of Figure 3.