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N. Moutalbi et al. / Physica C 471 (2011) 97–103
served to form effective flux pinning centers when added to the
(RE)-BCO matrix [12,13]. However, the performance level of HTS
rapidly decreases with an increasing field and temperature. It has
been proposed that the resulting pinning is due to the interface
or the point- like pinning centers, such as the lattice defects around
the embedded nano-particles. In spite of all these technological
and scientific challenges in cuprates, the dependence of the pin-
ning strength on the size of the pinning centers and the related ef-
fects on the superconducting properties are much less studied.
Indeed, it has been recently shown that a great improvement of
Jc in YBCO films is obtained with BaZrO3 or BaSnO3 nanorods
[14,15]. It has been found that the Jc–B properties vary depending
on the nanorod structure and that the pinning mechanism could be
changed by systematically controlling the nanorod length.
Since in a homogeneous type-II superconductor, the best size of
the pinning centers [diameter d ꢁ n(T)] is known [16], it is widely
believed that n(T) or twice n(T) is the best artificial pinning center
size. This straightforward extrapolation of the results obtained for
a homogeneous superconductor without pinning centers onto a
superconductor with artificial pinning centers meets with serious
controversies, since in many cases the presence of relatively large
pinning centers also leads to a strong pinning. Therefore, some the-
oretical studies based on Ginzburg–Landau theory suggested that
the optimal size of a single insulating inclusion acting as a pinning
center should be comparable to the magnetic penetration depth
rather than the coherence length [17–20]. Likewise, Matsushita
[21] reported that flux pinning should increase as the surface area
of the defect increases; e.g. if the insulating inclusion size in-
creases. At present, the theoretical understanding of the size
dependence of the pinning strength is still insufficient. Moreover,
these theoretical predictions have not been experimentally con-
firmed yet. To our knowledge, there is still no report on the depen-
dence of the pinning strength on the size of the pinning centers.
One of the reasons is that most studies focused mainly on achiev-
ing high densities of defects using optimal amounts of nano-metric
particles with a size and distribution which were not properly
controlled.
into pellets of 13 mm in diameter and 2 mm in thickness. The pel-
lets were sintered at 950 °C for 8 h in air and then cooled down to
room temperature. The crystalline structure was investigated by
powder X-ray diffraction (XRD) analysis. The X-ray diffraction
(XRD) patterns of ZrO2-added samples with various contents less
than 0.1 wt.% show that the entire major peaks of the Y123 ortho-
rhombic structure are identified, indicating that the samples
mainly consist of the desired Y123 phase. These results indicate
that ZrO2 nano-particles, which were used in little fraction in the
whole sample had not reacted with Y123 compound. The reflection
peak of ZrO2 could not be detected by XRD. The grain morphology
and microstructure analysis were carried out with a scanning elec-
tron microscopy. A dc magnetometer was used to measure the
magnetization loops with the magnetic field parallel to the c axis
of the specimen. Subsequently, the Bean critical state model [23]
is relied onto determine the magnetic critical current density Jc
from the measured magnetic hysteresis. The magnetic, Jc values
were thus estimated with the following equation: Jc = 20DM/
(a(1 ꢀ a/3b)), where
DM is the magnetization hysteresis in
emu cmꢀ3, a and b (a < b and both in cm) are the dimensions of
the sample in the plane perpendicular to the applied field. As for
D
M, we used an average of the width for positive and negative
magnetic fields. This approach provides a measure of the critical
current density since the inter-grain critical current density is usu-
ally much smaller than that for intra-grain. Furthermore, the cur-
rent–voltage (I–V) curves were measured by the four probe
method in liquid nitrogen (T = 77 K). Rectangular samples with a
cross section of about 0.04 cm2 were carefully cut from the pellets.
Current contacts on samples were done with commercial silver
paste. The transport critical current (Ic) and transport critical
current density (Jc) were estimated from the I–V curves with a
criterion of 5 lV/cm.
3. Results and discussion
In the present study, three batches of ZrO2 nano-particles with
different particle size distributions were used. Nano-sized ZrO2
The superconducting properties and the microstructure of
doped bulk superconductors with additions of ZrO2 nano-particles
have been intensively investigated [10,22,11]. High critical current
density was achieved at 77 K and further pinning could also be en-
hanced at high temperature and under high magnetic field. This
has demonstrated that the potential of ZrO2 nano-particles to-
gether with its simple compounds play an important role in the
enhancement of the effective vortex pinning. However, the effect
of ZrO2 addition on the performance of bulk (RE)-BCO is still con-
troversial. It was reported that the Tc of an Nd-BCO system with
added ZrO2 decreased compared to the un-doped composition
[22]. It was also argued that the ZrO2 addition leads to significant
pushing phenomena of Y211 particles in YBCO bulk superconduc-
tors while Jc values were enhanced with optimum doping amounts
[11]. In this work, we present a study of the influence of ZrO2 nano-
particles on the superconducting properties and performances un-
der magnetic field of bulk YBCO. The purpose of this study is to
investigate the dependence of the pinning strength on the size of
embedded ZrO2 nano-particles.
particles were synthesized by spray-pyrolysis assisted by
a
850 kHz ultrasonic atomizer starting from a precursor solution of
zirconyl nitrate (Fluka) at 2.5 ꢂ 10ꢀ2 mol Lꢀ1 concentration [24].
The aerosol was typically pyrolysed at 900 °C with a 3 L minꢀ1 flow
rate. Then, the resulting powder was heated in air for several suc-
cessive thermal treatments in order to favor crystallite growth. The
crystallite size and particle morphology were reported to extre-
mely depend on the spray-pyrolysis processing parameters. The
first zirconia batch is pyrolysed at 900 °C and the resulting powder
is characterized by 13 nm average crystallite size. The second and
third batches is heated after pyrolysis at 950 °C and 1400 °C and
are characterized by 21 nm and 85 nm average crystallite sizes,
respectively. The crystallite sizes were calculated from powder X-
ray diffraction measurements using the Debye–Scherrer formula.
Crystallite morphology of elaborated powders was examined using
transmission electron microscopy [25].
Fig. 1 shows scanning electron microscopy (SEM) micrographs
of ZrO2 nano-particles used in this study. Three different morphol-
ogies have been clearly observed. The first batch in Fig. 1a consists
of smooth and perfectly spherical particles. Rougher spherical par-
ticles are observed for the second batch in Fig. 1b. The particle mor-
phology of the third batch shown in Fig. 1c is quite different;
particles appear smoother and more distorted. Indeed, it is assumed
that zirconia powder prepared by spray-pyrolysis crystallizes into
the tetragonal phase as dense and compositionally homogeneous
polycrystalline spheres [25]. The pure tetragonal crystalline phase
was characterized by a crystallite size of 13 nm (first batch) whereas
monoclinic phase appears for 22 nm crystalline size (second batch)
2. Experimental
Y1Ba2Cu3Oy (Y123) powder was prepared by the conventional
solid state reaction of Y2O3, BaCO3 and CuO powders of 99.99% pur-
ity. The oxide mixture in the stoichiometric Y123 composition was
thoroughly ground and calcined at 950 °C for 24 h for two times.
Then, x percent in weight (xwt.%) of nano-ZrO2 particles were
added to the Y123 powder. The resulting mixtures were pressed
Moutalbi
