BaTiO3 powders doped with Ag at different Ag/Ba molar ratios were prepared by sol-gel method. The resistivity reached the lowest point of 5.644 Ω
Conductive powders are promising materials applied as film conductors [
BaTiO3 with a perovskite structure is noteworthy for its exceptional dielectric, piezoelectric, electrostrictive, and electrooptic properties with corresponding electronic applications [
In this paper, BaTiO3 powders doped with Ag were synthesized and low resistivity of such powders is reported. Their electrical and compositional characteristics are measured and studied.
The Ag-doped BaTiO3 powders (BATO) were prepared by sol-gel method [
The resistivity of the BATO powders was determined by using a standard four-point method (Keithley’s SourceMeter, model 2400, America). Structural analysis was performed by X-ray diffraction analysis (Seifert Debye Flex 2002). Fourier Transform Infrared spectroscopic measurements were performed by using an IR spectrophotometer (Nicolet AVATAR 320, America) ranging from 450 to 4000 cm−1.
The electric resistivity of BATO powders at different Ag concentrations is shown in Table
Electric resistivity of BATO powders before and after SCalcination (Ω
Process | |||||
0.0005 | 0.0010 | 0.0015 | 0.0020 | 0.0025 | |
Pro-SCalcination | 146.47 | 102.43 | 1821.04 | 2849.88 | 260.35 |
Post-SCalcination | 72.39 | 5.64 | 7.18 | 8541.82 | 2113.58 |
The comparison of the resistivity of BATO powders between (a) pro-SCalcination and (b) post-SCalcination.
The resistivity of undoped BaTiO3 powder is 4.0 × 109 Ω
It can be seen that as Ag concentration increased, the resistivity at first decreased, followed by an increase and another decrease. Such flat S-shaped trend was complied by the resistivity of both of pre- and post-SCalcination. The difference between the two processes is that SCalcination decreased the resistivity of BATO powders from the Ag/Ba molar ratio of 0.0005 to 0.0015, while it increased the resistivity at 0.0020 and 0.0025. The resistivity after SCalcination decreased the most at the Ag concentration of 0.15 at%, from 1821.04 Ω
Figure
XRD patterns of the BaTiO3 powders doped with Ag (1#) undoped BaTiO3; (2#) BATO with Ag concentration of 0.15 at%; (3#) after the calcination at 500°C for 2.5 h.
The particle size of BATO powders was calculated using Scherrer Formula and FWHM of (200) reflection observed via the X-ray data. The particle size of sample 1# is calculated to be 11.81 nm, while the particle sizes of sample 2# and 3# are 20.31 nm and 18.29 nm, respectively. In other words, we can see that the particle size was increased after doping but reduced after the SCalcination.
The FTIR spectra of samples 1#, 2#, and 3# are shown in Figure
FTIR spectrum of BATO powders: (1#) undoped BaTiO3; (2#) BATO with Ag concentration of 0.15 at%; (3#) after the calcination at 500°C for 2.5 h.
The characteristic absorption at 3410 cm−1 is assigned to–OH stretching vibration, due to the water brought by KBr or absorbed on the powder surface. The characteristic absorption at 1440 cm−1 is assigned to the stretching vibrations of carboxylate. This is because there is a small amount of BaCO3 in the samples, which is in agreement with the XRD patterns. All three samples exhibit strong absorptions around 550 cm−1 and 450 cm−1, which can be assigned to the stretching and bending vibrations of the Ti-O bond in [TiO6]2− octahedron. It is also a characteristic absorption of BaTiO3. But the wave number of the strongest absorption around 550 cm−1 varied slightly for the three samples: 547.8 cm−1 for sample 1#, 551.8 cm−1 for sample 2#, and 565.7 cm−1 for 3#. Since the wave number increases when infrared light of higher frequency and thus stronger energy is absorbed, it can be concluded that the Ti-O bond was strengthened after the doping and further strengthened after SCalcination.
During the calcination process AgNO3 decomposed into Ag2O, which entered BaTiO3 lattice. Since Ag+ possesses only one positive charge and Ba2+ possesses two, the substitution must be charge-compensated to maintain charge neutrality. According to the defect theory established by Kroger and Vink forty years ago, the incorporation of Ag2O as an acceptor dopant can be written as (
Since the perovskite structure is relatively a close packed one, the formation of interstitial ions is not as easy as the formation of vacancy. So the incorporation of Ag2O would prefer the way presented in (
The formation of oxygen vacancy can be proved by the FTIR result, in which the wave number of the characteristic absorption of Ti-O bond changed among the three samples. As stated previously, the strength of Ti-O bond in the [TiO6]2− octahedron was stronger in the sample doped with Ag than the undoped BaTiO3 sample. It can be explained as that, since the formation of oxygen vacancies means that there is O2− leaving the cell, the [TiO6]2− octahedron was distorted and Ti4+ gained a stronger attraction of the O2− left, rendering the Ti-O bond stronger.
The oxygen vacancy formed was in equilibrium with the formation of hole:
So that is the ultimate reason why doping AgNO3 would render BaTiO3 semiconductive.
As is shown in Figure
However the following drop of resistivity at Ag concentration of 0.20 at% and 0.25 at% may result from the formation of silver during the drop of temperature in the cooling process after calcination of the BATO powders. The silver would remain in the grain boundary and serve as a good conductor, reducing the grain boundary resistivity. Since the total resistivity of BaTiO3 is composed of grain resistivity and grain boundary resistivity, the overall resistivity is reduced. This explains the drop of resistivity after Ag concentration of 0.15 at% in Figure
To explain the conductivity trend presented in Figure
However at higher Ag concentrations (0.20 at% and above) the resistivity is higher after SCalcination than before. This may be due to two reasons. Firstly, as discussed previously, when the concentration of defect mounts to certain level, it will damage the conductivity. So SCalcination would worsen such process. Secondly, SCalcination would oxidize the silver formed during the cooling process after the first calcination.
BATO conductive powders were prepared by sol-gel method. The lowest resistivity of
XRD and FTIR investigations and measurements of electric conductivity combined proved that Ag+ entered the lattice of BaTiO3 and substituted for Ba2+, forming oxygen vacancies, which are the main reasons for the semiconductivity.
This work was supported by the National Science Foundation in China (no. 20571020).