TECHNICAL FIELD

The present application relates to a dense lead metaniobate (PbNb₂O₆) piezoelectric ceramic material and its preparation method, which belongs to the preparation field of piezoelectric ceramic materials.

BACKGROUND

Piezoelectric ceramic material is one of the most important functional materials for realizing electromechanical energy conversion and coupling, and has been widely used in electronic information, energy detection, advanced manufacturing, medical instruments, and weapon equipment, etc. Lead metaniobate (PbNb₂O₆) piezoelectric ceramic material, the first discovered ferroelectric material with a tungsten bronze crystal structure, is characterized by an extremely low mechanical quality factor (Q_m<10), single vibration mode (kt>>kp), high Curie temperature (Tc: ˜570° C.), and low depolarization at higher temperature, and has extensive and important applications in the fields of high temperature sensors, ultrasonic transducers, nondestructive testing, human ultrasound imaging, and so on.

The PbNb₂O₆piezoelectric ceramic material can only form a ferroelectric phase at a high temperature (˜1230° C.), but it presents a metastable state at room temperature. In a high-temperature sintering and cooling processes, an abnormal growth of PbNb₂O₆piezoelectric ceramic grains and rhombohedral-tetragonal phase transformation tend to cause great changes in volume and internal stress, resulting in a low density (relative density is about 80%) of PbNb₂O₆piezoelectric ceramics and sample cracking. Therefore, how to achieve high densification and avoid sintering cracking are the first issues to be concerned with in order to fabricate PbNb₂O₆piezoelectric ceramics with excellent performance.

At present, methods such as quenching, doping, or adding oxides are commonly used to inhibit the abnormal growth of grains and increase the density of PbNb₂O₆ceramics. Although the quenching method can restrain abnormal grain growth, it is not suitable for mass production due to its complexity. By replacing Pb′ with Ca′ and Ba′, and adding rare earth oxides and other doping modification methods, such as Pb_(1-x)Me_xNb₂O₆+Yy (Chinese Invention Patent ZL93112369.0,ZL00127914.9), xPb_1-m-n(La_m.Sr_n)Nb₂O₆+y(y₁PbTiO₃+y₂BiScO₃)+pwt % CeO₂+qwt % SiO₂+uwt % Ta₂O₅(Chinese Invention Patent ZL201210516363.1), the processing feasibility of the above-mentioned materials were improved, and piezoelectric properties were greatly enhanced, but the Curie temperatures Tc were significantly reduced.

SUMMARY

In order to solve the technical bottlenecks of cracking during the sintering process and low relative density of PbNb₂O₆piezoceramics, while maintaining the advantage of a high Curie temperature Tc, the present application provides a method of fabricating dense lead metaniobate piezoelectric ceramics.

The chemical composition of the as-mentioned lead metaniobate piezoelectric ceramic is Pb_1-xNb₂O₆, in which x represents the vacancy concentration of A-site Pb in a tungsten bronze crystal structure, and x is greater than 0.00 and less than or equal to 0.20.

In the present application, by introducing a certain proportion of Pb vacancy in composition design, the phase transformation occurring at about 1,230° C. during a high-temperature sintering process of lead metaniobate ceramics can be avoided, and the high volume and internal stress changes caused by abnormal grain growth and phase transformation can be inhibited, thus solving the technical problems of sintering cracks and low density of PbNb₂O₆piezoelectric ceramics.

Preferably, x is greater than or equal to 0.03 and less than or equal to 0.20. More preferably, x is greater than or equal to 0.08 and less than or equal to 0.20. Furthermore preferably, x is greater than or equal to 0.15 and less than or equal to 0.20.

Preferably, the lead metaniobate piezoelectric ceramic has a relative density of 93% to 99%, a shrinkage of 15% to 18%, and a Curie temperature Tc of 554 to 560° C.

Preferably, the lead metaniobate piezoelectric ceramic has a Curie temperature Tc of 554 to 560° C. and dielectric loss (tan δ) of 0.55% to 0.63%.

In another aspect, the present application provides a preparation method for the lead metaniobate piezoelectric ceramic, comprising the steps of: weighing raw compounds corresponding to each element according to the stoichiometric ratio of chemical composition of Pb_1-xNb₂O₆and then mixing uniformly to form a mixture; calcining the mixture at 800 to 900° C. for 1 to 3 hours to form a calcined powder; and sintering the calcined powder at 1,260 to 1,300° C. for 2 to 4 hours to obtain the lead metaniobate piezoelectric ceramic.

Preferably, the sintering temperature is 1,260 to 1,280° C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a scanning electron microscope image of lead metaniobate (PbNb₂O₆) piezoelectric ceramic material (x=0) obtained from comparative example 1;

FIG. 2 is a scanning electron microscope image of lead metaniobate (Pb_1-xNb₂O₆) piezoelectric ceramic material (x=0.03) obtained from example 1;

FIG. 3 is a scanning electron microscope image of lead metaniobate (Pb_1-xNb₂O₆) piezoelectric ceramic material (x=0.08) obtained from example 3;

FIG. 4 is a scanning electron microscope image of lead metaniobate (Pb_1-xNb₂O₆) piezoelectric ceramic material (x=0.15) obtained from example 5; and

FIG. 5 is a scanning electron microscope image of lead metaniobate (Pb_1-xNb₂O₆) piezoelectric ceramic material (x=0.20) obtained from example 7.

DETAILED DESCRIPTION

The present invention will be further described with the following embodiments below. It should be understood that the following embodiments are only used for explaining this invention, and do not limit this invention.

The present application provides a kind of PbNb₂O₆-based piezoelectric ceramic with high Curie temperature and high density. The chemical composition of the lead metaniobate piezoelectric ceramic is Pb_1-xNb₂O₆, wherein x is greater than 0 and less than or equal to 0.20, and represents the Pb vacancies concentration of A sites in a tungsten bronze crystal structure. The vacancy design of Pb in this application makes the composition of the material deviate from its stoichiometric ratio, but the main phase remains PbNb₂O₆, hence the Curie temperature Tc remains almost unchanged, though a small amount of the secondary phase is generated. The application has the following advantageous effects: on the one hand, it can promote liquid phase sintering at a high temperature, and make the grain size more uniform (especially when x is more than 0.05, liquid phase produced by high-temperature sintering can be seen more clearly at grain boundary from grain morphology images), and correspondingly, the density can also be improved; on the other hand, the phase transformation at 1,230° C. is inhibited, and the surface cracks caused by changes in volume and internal stress due to phase transformation are reduced or gradually eliminated, which promote the increase of density and reduction of cracks, further reducing the dielectric loss of PbNb₂O₆piezoelectric ceramics slightly.

In the chemical composition, x can be greater than or equal to 0.03 and less than or equal to 0.20. In some embodiments, x is greater than or equal to 0.08 and less than or equal to 0.20. More preferably, x is greater than or equal to 0.15 and less than or equal to 0.20. As can be seen from the SEM surface morphology, when the Pb vacancy concentration is greater than or equal to 0.08, the grain size changes significantly, specifically, the grain size decreases significantly and becomes more uniform; when x is greater than or equal to 0.15, these phenomena are more significant, that is, grain size is more uniform, and phase transition at 1,230° C. is completely inhibited, resulting in a further increase of densification and fewer surface cracks. It should be noted that when x is greater than 0.08, i.e., the composition design of the lead metaniobate piezoelectric ceramics deviates far from the stoichiometric ratio of the formula provided by the present application, it is easy to produce a large number of secondary phases or even multiphases. Hence, in that case, the main phase gradually loses its inherent characteristics of PbNb₂O₆piezoelectric ceramics, and has no practical value.

The following is an illustrative description of the preparation method of the lead metaniobate piezoelectric ceramics provided by this application.

Firstly, raw compounds of each element are selected as raw materials according to the chemical general formula of Pb_1-xNb₂O₆. In the formula, for example, the raw compound corresponding to Pb is selected from PbO or Pb₃O₄; and the raw compound corresponding to Nb is selected from Nb₂O₅.

Secondly, the raw materials are weighed and ground into a powder. For example, both raw materials are weighed according to the stoichiometric ratio of the chemical formula of Pb_1-xNb₂O₆, and then mixed evenly after adding deionized water, subsequently followed by grinding for 4 to 6 hours through planetary ball milling until the raw materials are well-mixed. In addition, the ratio of the weight of added deionized water to the total weight of raw materials is 125% to 135%.

Then, a solid-phase reaction is conducted to synthesize a calcined powder. The ground powder is subjected to a solid-phase reaction in air. The heating rate is 1 to 3° C./min, the reaction temperature is 800 to 900° C., and the reaction time is 1 to 3 hours.

Subsequently, deionized water is added to the calcined powder, followed by uniform mixing and grinding for 4 to 6 hours. The ratio of the weight of the added deionized water to the weight of the calcined powder is 125% to 135%.

Next, the ground powder is sintered in air, and the sintering method is solid-phase reaction sintering (also referred to as secondary solid-state reaction). The heating rate is 1 to 3° C./min, the reaction temperature is 1,260 to 1,300° C., and the reaction time is 2 to 4 hours.

Preferably, secondary solid-state reaction temperature is 1,260 to 1,280° C., and the reaction time is 2 hours.

In the present application, the Curie temperature Tc of the ceramic was obtained by measuring a dielectric temperature spectrum of a sample to be tested, using a precision impedance analyzer (LCR-HP4284A) connected with a high temperature dielectric temperature spectrum testing system (GJW-1, Institute of Electronic Materials, Xi'an Jiaotong University), in accordance with “Test methods for the properties of piezoelectric ceramics—Test for Curie temperature Tc” (GB3389.3-2001). The test temperature ranged from room temperature to 650° C., the heating rate was 2° C./min, and the test frequency was 1 kHz.

In the present application, the dielectric loss tangent (tan δ) was measured using a LCR digital bridge in accordance with “Test methods for properties of structure ceramic used in electronic components” (GB5594.4-1985). The test frequency was 1 kHz.

In the present application, the density was measured according to the Archimedes principle and calculated by the formula ρ=m₀*ρ_water/(m₁−m₂₎). In the formula, m₀is the mass of dried sample in air, m₁and m₂are respectively the mass of sample in air and water after fully absorbing water, and ρ_watercorresponds to the density of distilled water under the test environment. The calculation formula of relative volume density is: D=ρ/ρ₀*100%, where ρ and ρ₀represent the bulk density and theoretical density of the sample, respectively. The sample was heated in water bath at 150° C. for 4 hours, and m₁and m₂were measured by an electronic balance. Then the sample was dried at 120° C. and m₀was measured. Finally, the density was calculated.

Compared with other composition designs adopting different doping or adding oxides, the present application adopts an easier composition design of directly introducing Pb vacancy on the basis of the stoichiometric ratio and the principle of phase equilibrium, which can inhibit the abnormal grain growth and rhombohedral-tetragonal phase transformation at high temperature, significantly increase the relative density from 92% to 98%, significantly increase the sintering shrinkage ratio, and basically eliminate the surface cracks of ceramics, all while the Curie temperature and dielectric loss remain basically unchanged. Based on the above beneficial effects, the piezoelectric properties (such as a piezoelectric coefficient, electromechanical coupling coefficient, and mechanical quality factor) can be adjusted by doping appropriate ions in order to satisfy the requirements of different applications like sensors and transducers.

Experimental methods used in the following examples are conventional methods unless otherwise specified.

Materials and reagents used in the following examples can be obtained commercially unless otherwise specified.

TABLE 1

Comparison of sintering temperature and basic properties of Pb_1−xNb₂O₆

with different compositions

Curie

Sintering

Temper-
Dielectric

Temper-
Theoretical
Volume
Relative
Shrinkage
ature
Loss

ature
Density
Density
Density
Ratio
T_C
Tan δ

Number
x
(° C.)
(g/cm³)
(g/cm³)
(%)
(%)
(° C.)
(%)

Comparative
0
1260
6.621
6.102
92.2
14.68
557
0.66

Example 1

Example 1
0.03
1260
6.541
6.087
93.1
16.32
557
0.60

Example 2
0.05
1270
6.481
6.055
93.4
16.56
556
0.63

Example 3
0.08
1270
6.390
6.026
94.3
17.29
555
0.61

Example 4
0.10
1270
6.329
6.019
95.1
16.89
554
0.59

Example 5
0.15
1280
6.178
6.013
97.3
16.50
558
0.55

Example 6
0.18
1280
6.087
5.934
97.5
15.83
559
0.60

Example 7
0.20
1280
6.027
5.914
98.1
15.86
559
0.59

Comparative Example 1

The preparation method of a lead metaniobate piezoelectric ceramic comprises the following steps:

Step (1): weighing 46.565 g of PbO and 54.000 g of Nb₂O₅and placing the materials in a ball mill tank, and then adding 136 g of deionized water; adjusting the rotation speed of the ball mill to be 360 RPM (revolutions per minute), and continuous ball-milling for 4 hours to give an evenly mixed mixture;

Step (2): drying the mixture at 120° C. until it forms a gully shape and calcining the mixture at 850° C. for 2 hours in a low-temperature electric furnace; after grinding the calcined product to powder, passing the powder through a 40-mesh sieve to obtain calcined powder;

Step (3): sintering the prepared calcined powder at 1,260° C. for 2 hours in a high temperature furnace; and