Nondestructive testing of defects in adhesive joints

2.2. Characterization of electrospun P(VdF-
HFP) nanocomposite membranes
The thermal properties of the membranes
were evaluated by DSC (2010 TA Instruments)
at a heating rate of 10 ºC/min under a nitrogen
atmosphere from 50 to 200 ºC. The mechanical
properties were evaluated following ASTM
D638. The fiber morphology was recorded on
field-emission SEM (FE-SEM: Hitachi S-4800),
and the average fiber diameter (AFD) was
estimated. The porosity (P) [4] and tortuosity [5]
were determined as published elsewhere.
2.3. Electrochemical evaluation
PEs were prepared by soaking a circular
piece of the membrane (diameter 2 cm) in the
liquid electrolyte, 1 M LiPF6 in EC/DMC (1:1
v/v) (Samsung Cheil Industries Inc.). The
electrolyte uptake (δ) and the leakage properties
of the PEs were measured following the
procedure reported earlier [6].
The ionic conductivity of the PEs were
measured by the AC impedance method using
stainless steel (SS) Swagelok ® cells with 1M6
frequency analyzer over the temperature range
from 0 to 60 °C. The cell was kept at each
measuring temperature for a minimum of 30 min
to attain thermal equilibrium. The interfacial
resistance Rf between the PE and lithium metal
electrode was measured at room temperature by
the impedance response of Li/PE/Li cells. Both
the measurements were performed at an
amplitude of 20 mV over the frequency range 10
mHz to 2 MHz. The electrochemical stability
was determined by linear sweep voltammetry
(LSV) of Li/PE/SS cells at a scan rate of 1 mV/s
over the range of 2-5.5 V at 25 ºC. From the
porosity and conductivity measurements,
tortuosity of the membranes was calculated [5]:
Two-electrode lithium prototype coin cells
were fabricated by placing the electrospun PE
between lithium metal anode (300 μm thick,
Cyprus Foote Mineral Co.) and carbon-coated
lithium iron phosphate (LiFePO4) cathode [7].
The electrochemical tests of the Li/PE/LiFePO4
cells were conducted in an automatic
galvanostatic charge-discharge unit, WBCS3000
battery cycler (WonA Tech. Co.), between 2.5
and 4.0 V at 25 ºC at a current density of 0.1 C.
- 2 -
3. Results and discussion
3.1. Membrane morphology
SEM images of P(VdF-HFP) membranes
prepared without and with ceramic fillers reveal
the presence of well interconnected
interstices/pores between the fibers as shown in
Fig. 1. The use of the solvent mixture of
acetone/DMAc in the weight ratio of 7:3 results
in membranes with lower AFD as compared to
one component solvent or other solvent ratios
and smaller pore size due to the formation of
relatively large number of physical crosslinks.
The membranes have fully interconnected pore
structure. The ranges of fiber diameters obtained
for different samples along with the AFDs are
presented in Table 1. The membrane prepared
without ceramic filler exhibits a comparatively
uniform morphology with an AFD of 1.2 μm.
The AFD is higher for the membranes that
contain fillers. The membrane prepared with
BaTiO3 has more uniform fiber diameter and
narrower distribution of the fiber diameters as
compared to the other membranes. The larger
diameter of the fiber in membranes with fillers
can be attributed to the substantial increase in the
viscosity that results from the blending of
polymer solution with the filler particles.
Fig. 1. SEM images of electrospun P(VdF-HFP)
membranes with (A) no filler, (B) SiO2, (C) Al2O3,
and (D) BaTiO3.
3.2. Thermal and mechanical properties
The effect of incorporation of 6% nano-sized
ceramic fillers on the thermal properties of
electrospun P(VdF-HFP) polymer is shown in
Fig. 2. The melting temperature of pure P(VdF-
HFP) is 159 ºC [8], while the electrospun