1 In order to obtain ceramic capacitor materials with better dielectric functions, this article selects PM N-PT-BT (ie Pb (M g 1/ 3 N b 2/ 3) O 3-PbT iO 3-BaT iO 3) ternary system , And its doping modification. The selection basis of PM N-PT-BT system: PT is a known Curie point shift agent; BT is a good perovskite structure stabilizer in PMN.
2.1 Sample preparation
In this paper, the secondary pre-composition method proposed by SwartZ and Shrout is selected to form the PM N-PT-BT system. The basis is: the traditional oxide mixing method is used to prepare the pyrochlore phase, which often accompanies the deterioration of the dielectric function. PM N-PT-BT ceramics with perovskite phase structure, and the use of secondary pre-composition method can effectively suppress the occurrence of pyrochlore phase. The beginning of the study on changing the amount of PT and BT showed that 0.9875 <0.910PM N 0.090PT> 0.0125BT has the best dielectric properties in the entire series. PM N-PT-BT ceramics with perovskite phase structure, and the use of secondary pre-composition method can effectively suppress the occurrence of pyrochlore phase. When changing the amount of PT and BT, pre-compose MgNb 2 O 6 and then add PbO, TiO 2, BaCO 3, and dopants (each material shall be carried out in accordance with 0.9875 <0.910PM N 0. 090PT> 0. 0125 BT Chemical ratio, dopants are calculated separately), heat preservation at 800 for 3h to generate PMN-PT-BT system, after ball milling and drying, uniformly add 10% PV A as a binder, and press it with 15M Pa pressure A body with a diameter of 17mm and a thickness of about 3mm is heated at a rate of 300/h, sintered at a temperature of 1100 1250, and held for 22.5 hours. The surface is polished before the test, and a silver electrode is placed (electrode burning temperature 550).
2. 2 function test uses the capacitance C of the high voltage capacitor material of TH2613A capacitor at 1 kHz, and then calculates its dielectric constant according to r = 14. 4 Ch/ d 2 (h is the thickness of the ceramic sheet, d is the diameter) r, and measured the change of dielectric constant r with temperature. The dielectric loss tg is directly measured with TH2613A capacitance measuring instrument. Withstand voltage E b = breakdown voltage V (kV) / ceramic sheet thickness h (mm), the voltage source uses CH type (range 10kV), CH (range 20kV) type DC high voltage source. 3 Results and analysis 3.1 The effect of adding M gO and SrT iO 3 on the dielectric constant Adding MgO to the PM N-PT-BT system before sintering can completely improve the structure of the perovskite, because Mg 2+ is free In the system, the pyrochlore phase can be transformed into a perovskite structure. With the addition of MgO content, the Curie peak of the PM N-PT-BT porcelain body is broadened and moved to the low temperature direction, the dispersion is enhanced, and the curve becomes more and more smooth, and the dielectric constant temperature stability improves because of M gO The participation of the ceramic body can better change the perovskite structure of the porcelain body, but the dielectric constant decreases.
At this time, the sintering temperature of the porcelain body is relatively high, and the porcelain can be formed at around 1250. In order to make the dielectric constant not too small and have a higher compressive strength, first add 10wt% of MgO, and then add a certain amount of SrT iO 3 to obtain the dielectric temperature curve as shown in the figure. The participation of SrTiO 3 broadened the Curie peak more significantly and moved to the low temperature direction, the dispersion degree was greatly increased, and the medium temperature stability was greatly improved. Participating in the 7wt% SrT iO 3, has now approached a straight line. Compared with Pb (M g 1/ 3 Nb 2/ 3), the Curie temperature of O 3 is TC = 12 and the Curie temperature of SrT iO 3 is TC = 250. It is a paraelectric phase at room temperature. It can be incorporated into the system. Adding the ratio of the non-ferroelectric phase of the system can make the phase transition expansion of the system more significant, and the degree of dispersion can be added to further flatten the dielectric peak and reduce the C/C value. When 3wt% SrT iO 3 is added, the curve is smoother and the temperature stability is better. Its C/C (30 + 80) 15%. But the dielectric constant drops significantly. When participating in 7wt% SrT iO 3, the dielectric temperature curve is almost a straight line, but the dielectric constant at room temperature is less than 2000. In addition, after participating in SrT iO 3, the sintering temperature can be lowered, and porcelain can be formed below 1100.
3.2 The influence of adding M gO and SrT iO 3 on the pressure resistance
The change curve of the porcelain body breakdown field strength tested by placing the sample in trimethyl silicone oil is tested at room temperature (around 25). There are many factors that affect the breakdown of the sample, and the influence of the process is very direct. The following tests are all carried out under the same process conditions and environment. It can be seen that with the addition of the M gO content, the breakdown voltage of the porcelain body has a tendency to first increase and then decrease. The peak M gO content corresponds to 12. 5wt%, and the breakdown field strength at the peak is about 5900V. / mm. Add SrT iO 3 to the system with 10wt% MgO to obtain the change curve of the breakdown field strength of the porcelain body.
After adding 10wt% of MgO, and then adding a certain amount of SrTiO 3, the breakdown field strength fluctuates between the change curve of 4600V/mm 3 content of 4700V/mm. The addition of 10wt% M gO is about 4700V/mm, and the addition of SrT iO 3 does not significantly assist in improving the breakdown field strength of the porcelain body.
Because the breakdown of the sample mainly occurs at the edge of the electrode, in order to reduce this edge effect and further increase the breakdown field strength, an insulating varnish is applied to the edge of the sample. Participate in 3wt%SrT iO 3 in the system with 10wt% M gO added. After coating with insulating varnish, its breakdown field strength can reach 6. 2kV/ mm.
3.3 Effect of adding MgO and SrT iO 3 on dielectric loss With the addition of MgO, the loss is reduced, but it is not significant, and the curve is smooth. However, adding a small amount of SrTiO 3 reduces its sintering temperature, and it can be sintered at 1100 (without SrT iO 3, the sintering temperature is about 1250), so that PbO vaporizes relatively little, and there is a lack of lead in the unit cell. Relatively few, making domain movement more difficult. Because the domain wall movement is relatively difficult, the internal loss must be relatively reduced. With SrTiO 3 of about 3 wt%, the loss of the porcelain body is now in the order of 10-4.
Through the above experiments, it is found that the ceramic body function obtained by adding 10wt% MgO first, and then adding 3wt% SrTiO 3 is better, and it can be made into porcelain under 1100, = 5650
4.1 With the addition of M gO content, the dielectric constant of the porcelain body decreases, the Curie peak widens, moves to the low temperature direction, and the dispersion increases, and the breakdown field strength appears to be a modification process of first adding and then decreasing. When the content of M gO is 12.5%, the breakdown field strength reaches the maximum value of 5900V/ mm.
4.2 After adding 10wt% of M gO, following the addition of SrT iO 3 content, the dielectric constant further decreases, the Curie peak broadens and moves significantly to the low temperature direction, the dispersion degree is greatly increased, and the medium temperature stability is added .
4. 3 The participation of SrTiO 3 can lower the sintering temperature of the porcelain body, make the porcelain body become porcelain below 1100, and reduce the dielectric loss of the porcelain body at the same time, but it does not significantly assist in improving the pressure resistance.
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