Super Stable Ferroelectrics with High Curie Point.

Ferroelectric materials are of great importance in the sensing technology due to the piezoelectric properties. Thermal depoling behavior of ferroelectrics determines the upper temperature limit of their application. So far, there is no piezoelectric material working above 800 °C available. Here, we show Nd2Ti2O7 with a perovskite-like layered structure has good resistance to thermal depoling up to 1400 °C. Its stable behavior is because the material has only 180° ferroelectric domains, complex structure change at Curie point (Tc) and their sintering temperature is below their Tc, which avoided the internal stresses produced by the unit cell volume change at Tc. The phase transition at Tc shows a first order behavior which involving the tilting and rotation of the octahedron. The Curie - Weiss temperature is calculated, which might explain why the thermal depoling starts at about 1400 °C.

High-temperature piezoelectric sensing technology is of major importance for the chemical and material processing as well as automotive, aerospace, and power generating industries12. The commercial materials used for high temperature applications such as tourmaline (d33 ∼ 1.5 pC/N) can only work until 650 °C1345. Some ferroelectrics are potential for the high temperature applications due to their high Curie point (Tc), such as Aurivillius phase ferroelectrics678, Bi0.5Na0.5TiO3-based ferroelectrics91011, etc. However, all of them could not be used above 700 °C due to their thermal depoling behaviors67. The thermal depoling behaviors of ferroelectrics determine the upper temperature limit of their application as piezoelectrics. Thermal depoling is related to many factors, such as the phase transitions, ferroelectric domain structure, defects and internal mechanical stress78121314. For lead zirconate titanate ceramics, the internal stress and the non-180° domains reduced the thermal stability of their piezoelectric properties12. In the barium titanate system and (1−x)(BiScO3)−x(PbTiO3) compounds, the internal stress can seriously affect the thermal depoling stability1415. Defects in ferroelectrics can interact with domain walls and inhibit their movement at room temperature. However they can be thermally decoupled, which can produce thermal depoling13. In Aurivillus phase ferroelectrics, the non-180° domains could reduce the thermal stability78.

The ferroelectrics with perovskite-like layer structure (PLS) ferroelectrics show high Curie point (>1000 °C)16171819. In recent years, many investigations have focused on their potential to be used in high temperature piezoelectric sensor applications18192021. However, there is very limited information on the thermal depoling of PLS phase materials. In the present study, PLS ferroelectric ceramic, Nd2Ti2O7, was investigated and surprisingly it shows a super stable thermal depoling behavior up to 1400 °C. This opens a door for the development of new ferroelectrics for high temperature applications.

Results and Discussions

Fig. 1A shows the thermal depoling behavior of Nd2Ti2O7 ceramics, in which the piezoelectric constant d33, measured at room temperature, are plotted against the annealing temperature. The values of d33 of Nd2Ti2O7 samples (1–3) were measured as 0.5, 1.2, and 0.9 pC/N, respectively. The d33 value is very stable up to 1400 °C and drop to zero at about 1480 °C, which is the Curie temperature of Nd2Ti2O7. The variation of the d33 value is mainly due to the different poling conditions of these three samples. The sample 1, 2 and 3 were poled under the electric field as 20, 27 and 24 kV/mm, respectively. The difference of the poling electrical field is because of the different breakdown field of each sample. During the experiment, the thin ceramic sample was poled in the silicone oil at a temperature of 120 °C under an electric field. We increased the electrical field gradually from 10 kV/mm until the sample was electric breakdown. Therefore, the breakdown field of each sample decides the poling field, which can affect the piezoelectric activity. Compared to other ferroelectric compounds, the thermal depoling temperature of Nd2Ti2O7 is about 600 °C, 800 °C and 1000 °C higher than the values of Aurivillius phases, Bi0.5Na0.5TiO3- based compounds (BNT), and Pb(Zr,Ti)O3 compounds (PZT), respectively67813222324. This makes Nd2Ti2O7 a great candidate for the high temperature sensor applications, considering its d33 is acceptable, which is higher than the d33 of commercial piezoelectrics - tourmaline. Actually, this PLS ferroelectric material, Nd2Ti2O7, exhibits the highest temperature stability of piezoelectric properties among all known ferroelectrics so far as we known. The fact that PLS ferroelectrics only have 180° domains might explain the stability of these compounds. Nd2Ti2O7 has a monoclinic ferroelectric structure with P2 space group, and the lattice parameters is (a, b, c, β) = (13.020 Å, 5.480 Å, 7.680 Å, 98.3°) as shown in Fig. 2A212526272829. The paraelectric phase of Nd2Ti2O7, above the Tc, is orthorhombic lattice with the space group of Cmcm2728, shown in Fig. 2B. The ferroelectric spontaneous polarization (Ps) is induced by the rotation of the TiO6 octahedron around c-axis (blue arrow) and tilt around b-axis (red arrow), shown in Fig. 2A, which lead the Ps only in b-axis, producing 180° domains. In ferroelectric materials, the switching of non-180° domains produces a shape change3031. This can lead to large mechanical internal stress in poled materials. These internal stresses combined with thermal activition can produce thermal depoling. These effects are absent in materials with only 180° switching. Additionally, the ferroelectric to paraelectric phase transition of Nd2Ti2O7 involves all of the ions, from the TiO6 octahedron rotation and tilt. This characteristic might be another reason for its good stability and high Tc, due to that the complex structure change increase activation energy of phase transition. Furthermore, the sintering temperature of Nd2Ti2O7 ceramic (1350 °C), which is lower than the Tc (1481 °C), can reduce internal stress and increase thermal stability in this ferroelectric ceramic. Because sintering ceramics below its Tc can avoid the volume change associated with the Curie transition temperature on cooling from the sintering temperature.

Figure 1

Effect of thermal annealing on piezoelectric properties (d33).

Figure 2

The structure of Nd2Ti2O7 projected along the b- and c-axis for (A) ferroelectric phase (P21) and (B) paraelectric phase (Cmcm).

Figure 3A shows the spontaneous polarization (Ps) value of Nd2Ti2O7 based on the atomic displacements. Ionic displacements along the b-axis from the corresponding positions in the paraelectric structure produce the ferroelectric spontaneous polarization. Displacements along the a- and c- axes are cancelled due to the presence of centro-symmetric centers, which are therefore do not contribute to the total Ps. Based on the ionic displacements, the total Ps of ferroelectric Nd2Ti2O7 was calculated using Shimakawa’s model832:

Figure 3

(A) Ionic contribution to total spontaneous polarization (Ps) of each ion of Nd2Ti2O7. (B) The polarization – electric field plot (P–E) and current – electric field plot (I–E) measured at a frequency of 5 Hz.

where mi is the site multiplicity and Δxi is the atomic displacement along the c-axis, Qie is the ionic charge of the ion, and V is the volume of the unit cell. According to the crystal structure parameters of Nd2Ti2O7 reported on the single crystals23242526272829. the ionic contributions to the total Ps are presented in Fig. 3A, and the total Ps is calculated as 16.87 μC/cm2 at room temperature. Figure 3B shows the polarization – electric field plot (P–E) and current – electric field plot (I–E). From the un-saturated P-E plot, the Pr value is about 4.2 μC/cm2, which is much smaller than the value calculated. The coercive field is about 10 kV/mm.

Figure 4A shows dielectric constant (ɛ) of Nd2Ti2O7 ceramic as function of temperature at 1 MHz measured at heating and cooling processes at a rate of 5 °C/min. Typically, the dielectric constant peaks indicate the ferroelectric to paraelectric phase transitions (Curie point, Tc). The Tc at heating process is observed as 1481 °C which is in a good agreement with the literatures212233. However, the Tc for the cooling process was different from heating which is 1450 °C. The difference between heating and cooling suggests the ferroelectric to paraelectric transition of Nd2Ti2O7 has thermal hysteresis, which means this is a first order transition. This is also supported by the Curie - Weiss fitting. The function of 1/ε to temperature above Tc is shown in Fig. 5 according to the equations 2 and 3, where C is a material-specific Curie constant, T is the absolute temperature, and T0 is the Curie - Weiss temperature31.

Figure 4

(A) Temperature dependence of the dielectric constant of Nd2Ti2O7 at 1 MHz in the processes of heating and cooling. (B) The loss (tanθ) measured from 100 °C to 1500 °C at the frequency of 1 MHz, and the insert figure is the enlargement of range from 100 °C to 1100 °C.

Figure 5

Curie-Weiss fitting for the dielectric constant above the Tc for Nd2Ti2O7.

For Nd2Ti2O7, the slope of the fitting is 1.678 × 10−5, which is 1/C, and the intercept is the T0/C measured as 0.028, and the T0 is estimated as 1395 °C, lower than Tc in heating process. The T0 indicates the temperature point, at which the Ps starts going down, and this result might explain why the thermal depoling of Nd2Ti2O7 start at about 1400 °C. The loss (tan θ) at the heating process was shown in Fig. 4B and the insert figure is the enlargement of range from 100 °C to 1100 °C. The loss increase with the temperature increasing, and there is a broad peak just below the Tc, which can be attributed to ferroelectric domain wall movement. The loss below 1000 °C is small and this is helpful to develop the piezoelectric applications in the future.

In summary, perovskite-like layer structured ferroelectric, Nd2Ti2O7, has super high Curie points and stable piezoelectric properties. The stability of the piezoelectric properties can be explained by their stable ferroelectric domain structure, which consists of only 180° domains; the complex structure at Tc; and the fact that their sintering temperatures are below Tc, which avoids the internal strain produced by the volume change at Tc. The thermal depoling of Nd2Ti2O7 starts at about 1400 °C, which is the Curie - Weiss temperature, at which the Ps begin disappearing. This material has the potential to produce a step forward in the maximum operating temperature of ferroelectric/piezoelectric ceramics to >1000 °C.

Experiment Procedure

The starting materials were Nd2O3 (99.9% purity, Alfa Aesar) and TiO2 (99.95% purity, Alfa Aesar). The calcination conditions were 1250 °C for 4 h for powder synthesis. The ceramic was fabricated in a spark plasma sintering furnace (HPD 25/1, FCT Systems, Germany) using a two-step method56. The Nd2Ti2O7 powder was sintered at 1150 °C under 80 MPa for 3 min in a 20 mm-diameter graphite die firstly. Then the sintered ceramics were sintered at 1350 °C under 30 MPa for 5 min in a die with 30 mm diameter. X-ray diffraction (XRD, Siemens D5000, Karlsruhe, Germany) patterns was used to detect the phase of the ceramics. Electrodes were fabricated with fired-on platinum paste (Gwent Electronic Materials Ltd, C2011004D5) for electrical properties measurements. The temperature dependence of dielectric constant and loss was measured using a LCR meter (Agilent 4284A) connected to a tube furnace as shown in Fig. 6. The P–E and I–E loops were collected on the ferroelectric test module (TF Analyzer 2000 FE-module, aixACCT, Aachen, Germany). Samples for piezoelectric measurements were poled under various DC electric fields (20–30 kV/mm) in silicone oil at a temperature of 120 °C. We increased the electrical field gradually from 10 kV/mm until the sample was electrical breakdown to obtain high d33. Then their piezoelectric constant d33 was measured using a quasi-static d33 meter (CAS, ZJ- 3B) with the instrument precision of 0.1 pC/N17. To confirm the small d33 is not an experiment error, both sides of the sample was measured and the d33 are positive and negative on each side with the same absolute values. The d33 is zero on the un-poled samples as measured. The thermal depoling behavior was investigated by holding the samples at a fixed temperature for 4 hours, then measuring their piezoelectric constant after cooling.

Figure 6

The experimental set up used to measure permittivity and loss at high temperature.

Additional Information

How to cite this article: Gao, Z. et al. Super Stable Ferroelectrics with High Curie Point. Sci. Rep. 6, 24139; doi: 10.1038/srep24139 (2016).