3 Results and discussion
Fig.1 shows the XRD patterns of KNN powder
calcined at 900 "C and the ceramics sintered at 1 120
'C. The phase structure in all samples is pure perovskite
phase with typical orthorhombic symmetry and no any
secondary impurity can be certified. Fig.2 shows the
SEM micrograph of fracture surface of KNN ceramics.
The microstructure of the sample sintered at 1 120 'C is
uniform and fine, the grain boundary is clear, their
microstructure is dense, the average grain size is about 5
pm, the fracture surface is mainly intergranular fracture
as shown in Fig.2.
20 40 60 80 100
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Fig.1 XRD patterns of KNN powder calcined at 900'C(a) and
ceramics sintered at 1 120 'C(b)
Fig.2 SEM micrograph of kacture surface of KNN ceramics
sintered at 1 120 "C
Fig.3 shows d33 and kp values of KNN ceramics
sintered at 1 120 "C as a function of poling field with
poling time (20 min) and temperature(l00 'C) being
fixed as constants. It can be seen that the poling field
significantly affects the piezoelectric properties of KNN
ceramics, d33 and kp increase with poling field increasing
in the range of 2-4 kV/mm. However, when the poling
field exceeds 4 kV/mm, samples are electrically broken.
The poling field can cause domain switching and rotation,
make the ceramics exhibit piezoelectric properties.
Because the phase structure of KNN ceramics at room is
perovskite phase with orthorhombic symmetry, there are
60", 90", 120" and 180" domains in the orthorhombic
phase of KNN. During the course of the poling, the 180"
domain switching has no spontaneous strain, thus the 90",
120" and 180" domains can cause spontaneous strain,
consequently, the 180" domain can easily switch than
90", 120" and 180" domains. It can be observed from
Fig.3 that the increase of d3) and kpcan result from the
180" domain switching during lower poling field(