Study on Low Dielectric Loss and High Withstand Voltage BST Dielectric Ceramics

As a long-term problem in the research of pulse power technology, high energy storage density dielectric materials have always been a hot spot in pulse power technology research. It has been developing slowly for decades. Although some progress has been made in the late 20th and early 21st centuries, it is still difficult to meet the pulse. The need for power technology development. Currently, as a primary energy storage element in which the pulse power source using a dielectric material, such as titanate, niobate ceramics, a Mylar film still be of great concern scientific researchers.
Barium titanate is conventional ferroelectric ceramics dielectric storage material, present more of barium strontium titanate nanopowder research and ferroelectric thin film. However, in the case of bulk barium titanate, it has a perovskite ABO 3 type cubic structure between 120 and 1460 ° C, and has a tetragonal structure when the temperature is lower than 120 ° C. In the vicinity of the Curie temperature T C (T C =120 ° C), a phase transition occurs between the ferroelectric phase and the paraelectric phase. At this time, the material has a high dielectric constant, but at the same time its dielectric loss D and temperature coefficient α T increases. The greater the dielectric loss of a material, the weaker its ability to store energy and the greater its ability to dissipate energy. Dielectric loss is very harmful to the application of electronic materials and components in the circuit, because it not only causes additional attenuation on the line, but also causes components in normal equipment to heat up and stop working. In addition, the breakdown electric field strength of barium titanate ceramics is low due to defects in the material. The storage density of dielectric energy storage ceramics is proportional to the square of its compressive strength, so the storage density of barium titanate ceramics is low.
The purpose of this paper is to improve the energy storage dielectric properties of barium titanate-based ceramics. Attempts have been made to reduce the dielectric loss and increase the compressive strength of barium titanate ceramics by adding barium titanate and zinc oxide .
    First, the experiment
The Ba x Sr 1-x TiO 3 powder was prepared by hydrothermal method of nano-sized BaTiO 3 and SrTiO 3 powders by molar ratio x:(1-x). The purity and average particle diameter of the original BaTiO 3 and SrTiO 3 powders were 99.99% and 100 nm, respectively. The weighed raw powder was placed in a polyethylene bottle, and anhydrous ethanol and agate balls were mixed on the mixer for 16 hours. The mixed slurry is placed in an oven and dried, and then ground through a 100 mesh sieve with an agate to obtain a mixed powder of barium titanate and barium titanate. The above mixed powder was placed in a high temperature molybdenum disilicide resistance furnace to 4 ℃ · min - 1 was heated to a temperature rise rate of 1100 ℃, after 4h incubation furnace cooling, to obtain Ba 1-x Sr x TiO 3 powder. The above-mentioned calcined Ba x Sr 1-x TiO 3 powder was doped or undoped with ZnO, placed in an agate tank, and ball- milled on a planetary ball mill for 24 hours with anhydrous ethanol and agate balls.
The homogenized blank was obtained by cold-pressing a powder prepared by ball milling in a steel mold having a diameter of 21 mm, and then subjecting it to cold isostatic pressing at 2000 MPa (thickness: 3.5 to 4.5 mm). The obtained blank was sintered and densified by the sintering process shown in Fig. 1.
Fig.1 Sintering process curve of Ba 1-x Sr x TiO 3 ceramic sheet
The homogenized sintered wafer was cut to a thickness of 0.4-0.6 mm for the puncture strength test on a Model 30/20 high-voltage tester; the dielectric electrode was tested by E4294A after cutting a silver electrode having a thickness of 2 to 3 mm. The phase structure and microscopic morphology of the sintered samples were analyzed by X-ray diffractometry and scanning electron microscopy, respectively.
    Second, the results and discussion
(1) Phase structure
Figure 2 shows the results of XRD analysis of room temperature Ba x Sr 1-x TiO 3 . It can be seen that as the x increases, the unit cell parameters of the crystal also gradually increase, and the position of the diffraction peak is shifted to the left side sequentially as x increases. In addition, the lattice structure of the barium titanate solid solution is different, and is a tetragonal structure when x=1, and a cubic structure when x=0-0.6. This is because the Curie temperature of pure barium titanate is about 120 ° C, the para-electric phase is above the Curie temperature, the crystal structure is cubic structure; the ferroelectric phase is below 120 ° C, and the crystal structure is tetragonal. The Curie temperature of pure barium titanate is near absolute zero, and its crystal structure is cubic phase above the Curie temperature. When barium titanate and barium titanate form a Ba x Sr 1-x TiO 3 solid solution, the relationship between the Curie temperature and x is (temperature in K):
Figure 2 XRD analysis results of Ba x Sr 1 - x TiO 3
T C =400-220(1-x)-160(1-x) 2 (1)
According to the formula (1), the Curie temperature of Ba x Sr 1-x TiO 3 is already near room temperature (about 16 ° C) when x = 0.6, so when x is not more than 0.6, it is a cubic phase.
(2) Dielectric constant and dielectric loss of Ba x Sr 1-x TiO 3 ceramics
Figure 3 is a graph showing the relationship between the dielectric constant and the frequency of Ba x Sr 1-x TiO 3 ceramic. It can be seen that as x increases, the dielectric constant of Ba x Sr 1-x TiO 3 increases, and when x=0.5, the following relationship is basically satisfied:
Lnε r =V 1 lnε r1 +V 2 lnε r2 (2)
When x is greater than or less than 0.5, the measured dielectric constant of Ba x Sr 1-x TiO 3 is slightly higher or lower than the value calculated by the formula (2), respectively. This may be because ε r1 and ε r2 in the formula (2) represent the dielectric constants of two independent phases in the ceramic, respectively, and the barium titanate and barium titanate sinter form a single solid solution phase, not in two independent Phase exists. We calculate the V 1 and V 2 in the formula into the volume fraction of barium titanate and barium titanate, respectively, rather than the volume fraction of two separate phases in the ceramic. When the two components form a solid solution, the dielectric constant may be more biased than the result of the formula (2).
Fig. 3 Relationship between relative dielectric constant and frequency of Ba x Sr 1-x TiO 3
Figure 3 is a graph showing the dielectric loss versus frequency for Ba x Sr 1 - x TiO 3 ceramics. It can be seen that as x increases, the dielectric loss increases, and the loss value fluctuates greatly with the change of frequency. The D value is the tangent value of the loss angle - tan δ, which is an important one in dielectric materials. parameter. The relative dielectric constant ε r in Fig. 3 can be decomposed into two parts, ε ′ r and ε ′ r , as shown in Fig. 5 . Where ε ′ r is a measure of the ability of a dielectric material to store energy, and ε ′ r is a measure of the ability of a medium to dissipate energy. The expression of Tanδ is as follows:
Fig. 4 Relationship between dielectric loss and frequency of Ba x Sr 1-x TiO 3
Figure 5 Definition of loss tangent
Tanδ=D=1/Q=ε ′′ r /ε ′ =Energy lost per cycle/ Energy stored percycle (3)
Therefore, as an energy storage dielectric material, it is very important to reduce the dielectric loss.
(III) Relationship between dielectric properties of Ba 0.2 S 0.8 TiO 3 ceramics and the amount of ZnO added
ZnO was added in a mechanically mixed manner in Ba 0.2 S 0.8 TiO 3 powder in an amount of 0.0%, 0.4%, 0.8%, 1.2%, 1.4%, 1.6%, 1.8%, 2.0% (mass fraction), respectively. The dielectric properties and breakdown strength of the system were investigated. The results are shown in Table 1. The test frequency of ε ′ r and tan δ is 1 kHz, and the test signal of the breakdown voltage is a pulse signal.
Table 1 Effect of ZnO content on dielectric properties of Ba 0.2 S 0.8 TiO 3
ZnO%
0.0
0.4
0.8
1.2
1.4
1.6
1.8
ε r
437
439
443
448
451
456
391
Tanδ
0.0065
0.0068
0.0051
0.0032
0.0018
0.0016
0.0019
E b1 /(kv·mm -1 )
11.6
11.9
12.2
14.9
21.5
31.8
21.6
E b2 /(kv·mm -1 )
12.5
12.3
13.5
15.4
26.9
32.2
21.8
E b3 /(kv·mm -1 )
13.1
13.6
14.2
16.9
27.8
45.0
22.4
E b average/(kv·mm -1 )
12.4
12.6
13.5
15.7
25.4
36.3
21.93
It can be seen from Table 1 that as the ZnO content increases, the dielectric constant increases, the breakdown strength increases, and the dielectric loss decreases. When the ZnO content is greater than a certain value, both the dielectric constant and the breakdown strength of the material are reduced. This may be due to the fact that when ZnO is added in a small amount, it will solidify into the BST lattice at the beginning of sintering, since the ionic radii of Zn 2 + and Ba 2 + , Sr 2 + and Ti 4 + are quite different. As a result, the main crystal phase lattice is greatly distorted, so that the lattice energy is increased, the structural elements are easy to move, and sintering is promoted. Figure 6 shows the results of XRD analysis of Ba 0.2 S 0.8 TiO 3 and Ba 0.2 S 0.8 TiO 3 +1.6% ZnO. It can be seen that the addition of ZnO causes the (110) and (111) peaks of the BST sintered body to shift to the left. The unit cell volume increases, which may be caused by the substitution of Zn 2 + for Ti 4 + , because the ionic radius of Ti 4 + is r 6 = 60.5 pm, and the ionic radius of Zn 2 + is r 6 = 74 pm. The alternative meets the tolerance theorem. When the amount of ZnO added continues to increase, too much ZnO will precipitate as impurities or independent phases at the initial stage of sintering, and will be distributed at the BST grain boundary, thus hindering the movement of the BST grain boundary during the sintering process, hindering the grain. Developed so that the resulting sintered body is not dense enough. A ZnO powder having a mass ratio of 1.6% was mixed into the BST powder by mechanical mixing, and a ZnO diffraction peak was observed in the XRD analysis result, and the powder was pressed into a sheet and sintered at 1400 ° C for 4 hours. The presence of the second phase diffraction peak is not observed (as shown in Figure 7). This indicates that ZnO is solid-solved into the BST during sintering, or that although the second phase having a low melting point is formed, the second phase precipitates little at the grain boundary or dissolves into the BST grains at the later stage of sintering.
Fig.6 XRD analysis results of Ba 0.2 S 0.8 TiO 3 and Ba 0.2 S 0.8 TiO 3 +1.6% ZnO
Fig. 7 XRD analysis results of BST powder, sintered BST, BST+1.6% ZnO powder and sintered BST+1.6% ZnO
Fig. 8(a), (b) and (c) show the results of scanning electron microscopy analysis of Ba 0.2 S 0.8 TiO 3 sintered body with ZnO addition amount of 0.0%, 1.6% and 1.8%, respectively. The ceramic body obtained at the time of % is the most dense. It can be seen from the results of Table 1 that the dielectric constant and the breakdown strength of the material are also the highest, and the dielectric loss is the smallest. In addition, Figure 8 also shows that ZnO promotes the sintering and grain refinement of BST under the same sintering temperature and holding time.
Figure 8 ZnO additions are: (a) 0.0% (b) 1.6% (c) 1.8% SEM morphology of Ba0.2S0.8TiO 3 ceramics
    Third, the conclusion
(1) The cell size, dielectric constant and dielectric loss of Ba x Sr 1-x TiO 3 ceramics increase with increasing x, while the frequency stability of dielectric constant and dielectric loss The increase in x decreases.
(2) ZnO is added by mechanical mixing in Ba 0.2 Sr 0.8 TiO 3 powder. As the content of ZnO increases, the dielectric constant increases, the breakdown strength increases, and the dielectric loss decreases. When the ZnO content is greater than a certain value, both the dielectric constant and the breakdown strength of the material are reduced. When the amount of ZnO added is 1.6%, the obtained ceramic body is the most dense, and the dielectric constant and breakdown strength of the material are also the highest, and the dielectric loss is the smallest.
(3) A small amount of ZnO is added to the BST powder, and ZnO can be dissolved into the BST during the sintering process of the ceramic.

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