ABSTRACT: Microstructure, dielectric, piezoelectric and optic properties of Bi, Ni and Nb BNN doped

1
NANOSTRUCTURES PZT-BNN MATERIAL
WITH HIGH RELATIVE DIELECTRIC CONSTANT
E. DIMITRIU, C.GHICA, V. ALDICA, V.
GHIORDANESCU, R. RAMER National Institute for
Materials Physics, RO 76900, Bucharest-Magurele,
Romania University of South Wales, School of
Electrical Engineering, Sydney, Australia
ABSTRACT Microstructure, dielectric,
piezoelectric and optic properties of Bi-, Ni-
and Nb- (BNN) doped PZT were studied. Powders
were prepared by mixed-oxide method and different
sintering temperatures were used to obtain the
highest relative dielectric constant of about
4700. Typical TEM and SEM micrographs have been
obtained on specimens. XRD analysis shows the
presence of tetragonal phase with the calculated
lattice parameters a b 0.401nm and c
0.408nm. Dense piezoelectric ceramics (? 7.5
g/cm3), with Curie temperature over 200oC were
obtained. Other following characteristics were
obtained electromechanical coupling factor kp
between 0.4-0.5, piezoelectric coefficient (-d31)
of about 225 .10-12 C/N, frequency constant of
1450 Hz.m, mechanical quality factor of 40. The
behaviour of electrical resistivity at
temperatures between 20 and 500K was
investigated. Optical reflection spectra in
visible region were measured. The PZT-BNN ceramic
is a promising material for devices where high
permitivity is required.

• EXPERIMENTAL PZT-type ceramics doped with Bi, Ni
  and Nb were prepared by the usual ceramic
  technique involving the steps weighing the
  stoichiometric quantities of pure raw materials
  mixing in a planetary mill in agate vessel in
  methanol, for 2 hours calcining the dried
  mixture at 880oC for 4h, in dense alumina
  crucible milling the calcined product for 6h
  pressing the milled powder as discs sintering
  the samples at temperatures between 1180 and
  1230oC poling the samples in a silicon oil bath
  at 200oC under 3 kV/mm electric field.
• The dielectric and piezoelectric parameters were
  measured, 24 h after poling, by means of a HP
  4194A impedance/gain phase analyser.
• The electron microscopy images have been obtained
  with a JEOL 200CX electron microscope operated at
  200 kV with a 0.27 nm point resolution. The XRD
  spectrum has been acquired on a Seifert
  installation.
• The optical reflection spectra were measured in
  the visible region.
• The electrical resistivity was measured by
  standard four-probe method at temperatures
  between 20 and 500K, with a dc current density
  20nA/cm2 .

X-ray diffraction spectra reveals the formation
of a unique crystalline tetragonal phase, with
the calculated lattice parameters a b 0.401
nm and c 0.408 nm.
TEM image (a) and the corresponding SAED pattern
(b) of a crystal grain showing domains rotated at
85o.
The plot of electrical resistivity vs.
temperature
The behaviour of relative dielectric constant ?r
as a function of sintering temperature
The behaviour of coupling coefficients kp and kT
as a function of sintering temperature
Piezoelectric coefficients d31 and g31 as a
function of sintering temperature

• A PZT-type material doped with Bi, Ni and Nb was
  prepared by oxide-method.
• TEM micrograph shows a typical crystal grain of
  about 450 nm, resulted from the preparation
  procedure, made up of elongated domains visible
  as stripes of about 50 nm wide. The crystal grain
  is characterized by a typical contrast proving
  the existence of lattice strain.
• The corresponding selected area electron
  diffraction (SAED) confirms the formation of a
  well-crystallized tetragonal phase. The
  diffraction pattern is the result of the
  superposition of two diffraction patterns along
  the same 001 zone axis coming from domains
  rotated at 85 to each other. The rotation angle
  can be measured on the SAED pattern as well as on
  the TEM image as the angle between the domain
  walls.
• Dense piezoelectric ceramics (? 7.5 g/cm3),
  with Curie temperature over 200oC, and relative
  dielectric constant of about 4700 were obtained.
• Optical reflection R (?) spectra were measured in
  the visible region. The minima in reflection
  match the maxima in the absorption spectra. The
  absorption in the 550nm region can be due to the
  transition 2T?2E of titanium (III) and the peaks
  at 400, 470 and 680nm to the transitions 3F?3T2,
  3T1(F), 3T(P) of Ni (II). This optical behaviour
  is observed in the gap of the material. The value
  of gap energy is estimated at 3.6 eV.
• The metallic non-linear dependence of resistivity
  vs. temperature of the poled samples was
  observed. The polynomial fit is roughly accepted
  by this unusual behaviour. The metallic
  conduction in PZT-BNN ceramic may be attributed
  to x electrons, in a partially filled band of
  electron states. This conduction band, may be
  produced by a covalent bonding from a mixing of
  the oxygen p? orbitals and titanium and nickel
  transitions.

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AKNOWLEDGEMENTS POLECER Network and Romanian
National Program ORIZONT for the financial support