DE1104882B - Process for the production of a ceramic dielectric with high dielectric constant and low temperature coefficient of the DC - Google Patents

Description

Method of manufacturing a ceramic dielectric with high Dielectric constant and low temperature coefficient of the DK The invention relates to ceramics with a high dielectric constant, which at fairly strong temperature fluctuations remains practically constant.

It is known that they essentially consist of barium titanate Ceramics have high dielectric constants with relatively low losses. The use of these substances as dielectrics for capacitors is meanwhile limited that its Curie point is about 120 ° C and that near this point the dielectric constant s rises abruptly. These substances can therefore be used in cases where good stability of the dielectric constant s as a function of temperature required cannot be used.

Around this sudden increase in the dielectric constant near the Avoiding the Curie point, d. b. to achieve a dielectric constant / temperature curve that is as flat as possible Various compositions and various manufacturing processes have already been used to obtain suggested. So z. B. the addition of small amounts of magnesium titanate, calcium titanate, Strontium titanate, iron, lead, bismuth and calcium oxide, of alkaline earth fluorides, from alkali or cadmium niobates or tantalates to barium titanate, among others Conditions weaken an increase in s near the Curie point.

Certain known methods can achieve the same result: So you get z. B. by mixing certain amounts of titanates, their Curie points lying at different temperatures, and burning the mixture at such a low one Temperature that no reaction can take place between the components Material whose - values at the Curie point show only very slight anomalies; which scored However, the electricity constant is relatively low.

Those with the compositions mentioned above and after those described Substances obtained from the manufacturing process all have a common property; namely, they form macroscopically heterogeneous compositions in which chemically and crystallographically different crystals are adjacent.

In certain known compositions one can determine the presence a cubic perovskite phase with a Curie point below room temperature and a tetragonal barium titanate phase with a Curie point at 120 ° C. The mixture of these two phases results in a dielectric material with a very slight increase in s between 10 and 120 ° C.

In other known compositions two coexist Phases of different chemical nature and with different electrical ones Properties.

All of these known, heterogeneous substances have certain disadvantages: Their dielectric constant is usually below 1000 and fluctuates that are still too strong for certain purposes and within too narrow temperature limits (usually 25 to 85 ° C) occur. The main disadvantage these substances, which are a consequence of their heterogeneous nature, lie in theirs poor reproducibility and thus the difficulty of series production.

The starting materials, the production of the mixture (grinding, shaping etc.) as well as the temperature of the fire must be clearly defined as the the slightest fluctuation of one of these technological factors the predominance of one or the other phase and thus a change in the electrical properties of the End product brings with it. The manufacture of such products is therefore extreme difficult and the committee very large. After all, is the temperature of the fire these substances are usually very high (1300 ° C and above), which is expensive Ovens and requires a large amount of energy.

The invention now relates to compositions for ceramic dielectrics which do not have the disadvantages listed above. The compositions according to the invention contain barium titanate and 0.5 to 50 percent by weight of a composition or a mixture of the following compositions represented by the general chemical formulas (A "BiyB209), where A = Bi, Ba, Sr, Ca, Pb, Na, Ii, Cd denotes or more of these elements; B denotes Ti, Nb, Ta or their mixtures; yt is an integer and the sum of x and y is 3.

According to a particular embodiment, the composition has n (A "BiyB209) has the formula CdBi2NbxTa (2, t) O9, where rt is a number between 0 and 2 is.

Among the compositions with the given general formula may be mentioned: Bi3NbTi09 - CaBi2Nb209 - SrBi2Nb209 -BaBi2Nb209 - PbBi2Nb209 - KBi5Nb401 $ -NaBi5Nb4019 and CdBi2Nb20g.

According to the method according to the invention, the specific amounts the starting materials, which barium titanate or the selected compound na (AxBiyB209) result, pre-fired separately. The barium titanate and compound are then mixed n (A, BiyB209) in the proportions given above and subject to Shaping the body obtained in this way by means of a suitable binder Heat treatment at a temperature between 1100 and 130O ° C.

The invention can be better understood with reference to the drawing and the following, non-limiting examples. In the drawing, Fig.l shows the crystal structure of the compositions it (A, BiyB209), Fig.2 the curve of the dielectric constant s as a function of the temperature t (expressed in degrees C) for a ceramic based on barium titanate with a content of 15 ° / o Bi3NbTi09, Fig. 3 the curves of the dielectric constant s as a function of the temperature t for ceramics based on barium titanate with different percentages of CaBi2Nb209: curve (a): 90 g BaTi03 - (- 10 g CaBi2Nb209, curve (b): 85 g BaTi03 + 15 g CaBi2Nb209, curve (c): 80 g BaTi03 -f- 20 g CaBi2Nb209; Fig. 4 shows the curves of the dielectric constant a as a function of the temperature t for ceramics based on barium titanate with different percentages of SrBiNb209, Fig. 5 shows the same Curves of s as a function of t for ceramics based on barium titanate with different contents of BaBi2Nb209, FIG. 6 shows curves a as a function of t for ceramic dielectrics made of barium titanate with different percentages contained in PbBi2Nb209.

The curves (a), (b), (c) of FIG. 7 show the changes in the dielectric constant s as a function of the temperature t ('C) for ceramics based on barium titanate with various percentages of CdBi2Nb209.

Example 1 Barium titanate is made by burning a very homogeneous Mixture of titanium dioxide and barium carbonate in equimolar amounts at 1260 ° C and thorough grinding of the product obtained.

Bismuth oxide, niobium oxide and titanium dioxide are also burned in a molar ratio of 3Bi203, 1Nb205, 2Ti02 at a temperature of 1000 ° C. The product obtained in the process is also finely ground.

Then mix 85g of barium titanate and 15g of the one obtained in the standing room Titanium-Niobium-Bismuth Composition. The powdery mixture is mixed with a combustible organic binder and deforms the mixture in a known manner. The samples obtained are exposed to a temperature of 1200 ° C. for about 1 hour. It should be noted that the suitable temperature range of the fire is very wide. the end For economic reasons, however, the lowest possible temperature is preferred from Z. B. 1200 ° C, but always making sure that the products obtained are dense and show a very nice glassy fracture.

As the curve in FIG. 2 shows, the dielectric constant is of the product obtained at 30 ° C about 1800, and it varies between 10 and 80 ° C by less than ± 0.80 / ,. The dielectric loss angle tg a, measured at 1 kHz, is below 100. 10-4.

Example 2 1 mole of calcium carbonate CaCO2, 1 mole of bismuth oxide bit 03 and 1 mol of niobium pentoxide Nb2 05 is mixed thoroughly and fired at 1000 ° C for about 1 hour.

10 g of this composition are added to 90 g of the according to the procedure barium titanate obtained from Example 1 to.

After shaping, the mixture is fired at 1200 ° C. for about 1 hour.

The material obtained has essentially the same appearance like that described in the previous example. The change in dielectric constant as a function of temperature is shown in curve (a) of FIG. One can see that the dielectric constant s at 25 ° C is 1500 and varies between 25 and 85 ° C changes by a maximum of ± 2 ° / 9 and between -50 and -f-100 ° C only by 10 ° / a. The dielectric Loss angle is less than 100 - 10-4 at 30 ° C and 1 kHz.

The 15 and 20% CaBi2Nb209 containing materials have the dielectric properties shown in curves (b) and (c) of FIG. 3, respectively.

Example 3 Mixtures are prepared by following the procedures of Examples 1 and 2 from BaTi03 - SrBi2Nb209, with the composition SrBi2Nb209 for 1 hour 1000 ° C burns.

The curves showing the change in dielectric constant as a function of from the temperature are shown in Fig. 4 curve (a): 90 g BaTi03 + 10 g of SrBi2Nb209. Curve (b): 85 g BaTi03 + 15 g SrBi2Nb209.

It can be seen from curve (a) that the change in E between -50 and -f-130 ° C fluctuates by + 5% around an average value of 845; tg bl at 1 kHz = 50.10-4.

The curve (b) shows a change in s between -50 and + 150 ° C of only + 3 ° / a.

Mean value of a = 1170 tg 0 at 1 kHz and 20 ° C = 80.10-4.

Example 4 The procedures of the preceding examples are prepared Mixtures of BaTi03 - BaBi2Nb209, the composition BaBi2Nb209 1 hour is fired at 1000 ° C.

Fig. 5 shows the changes in dielectric constant of the obtained Product as a function of the temperature curve («,): 90 g BaTi03 + 10 g BaBi2Nb209. Curve (b): 80 g BaTi03 + 20 g BaBi2Nb209.

Curve (a): mean of s = 900; s changes by ± 5 ° / o between -50 and + 120 ° C; Value for tg 0 at 1 kHz = 90 - 10-4. Curve (b): mean value of a = 650; a changes by ± 7 ° / 9 between -50 and -E-130'C; Value for tg 0 at 1 kHz = 50 - 10-4.

Example 5 Following the procedures of the preceding examples, make mixtures of BaTi03 - PbBi2Nb209 are produced, the composition being PbBi2Nb209 Firing at 900 ° C for 1 hour.

Fig. 6 shows the changes in dielectric constant of the obtained Product depending on the temperature curve (a): 90 g BaTi03 + 10 g PbBi2Nb209. Curve (b): 85 g BaTi03 + 15 g PbBi2Nb209. Curve (c): 80 g BaTi03 -1-- 20 g PbBi2Nb2O9.

Curve (a): mean value of a = 1380; s changes by 4.3% between -60 and + 110 ° C; Value for tg 0 at 1 kHz = 90. 10-4.

Curve (b): mean value of a = 1100; e changes by ± 4.10 /, between -60 and + 120'C; Value for tg b at 1 kHz = 60 - 10-4.

Curve (c): mean of s = 920; a changes by ± 7.5% between -60 and -r 120 ° C; Value for tg ü at 1 kHz = 40 - 10-4.

Example 6 The products are produced in the same way as in the previous examples.

The compositions and electrical properties of the final product are summarized in the table below: Compositions of dielectric change constant from a tg 8 - 10-4 BaTi O CdBi, between at 20 ° C Weight Nb @ 0 ° at 20 ° C and --40 and and Weight- 1 kHz -I-100 C 1 kHz percent percent ° lo 90 10 1130 5 90 85 15 900 + 3; 5 66 80 20 710 ± 4.4 29 The substances according to the invention, and in particular those described in the individual examples, were prepared repeatedly, the proportions of the starting products and the manufacturing conditions varying, without a large scatter in the electrical properties of the products obtained being able to be observed as a result. Slight deviations in the molar ratios of the individual components can be permitted without the properties of the substances being disturbed within wide limits.

The remarkable properties of the dielectrics according to the invention explain themselves on the basis of the molecular structure of the compositions of the general Formula n (A #, BiyB209).

It is noted that the metal ions A have an ionic radius between 0.98 and 1.43 A ° and the ions B have an ionic radius of about 0.65 A °.

Figure 1, in which the open circles are oxygen atoms, the hatched Circles bismuth, the black circles B-ions and the one square hatching having circles can represent one or more bismuth or B ions, shows the regular change of groupings (B'202) ++ (denoted by K) with the Grouping (L) of the perovskite type. These latter groupings are analogous the compositions n (A "BiyB209) changeable. So they are in the composition CaBi2Nb209 represented by groupings (CaNb20 ,,) -. In the composition KBi5Nb40 "changes a grouping (B'202) - with a grouping (KBi), Nb20, ++ away.

2 This fact that these structures include groupings of the perovskite type explains that these compositions can form solid solutions with the barium titanate, which also crystallizes in the perovskite type, which in turn form a crystal structure of the perovskite type.

Claims (1)

PATENT CLAIM: Process for the production of a ceramic dielectric with a high dielectric constant and low temperature coefficient of the DC the basis of BaTiOS, characterized in that one of 50 to 99.5 percent by weight Barium titanate and 50 to 0.5 weight percent of one or more prefired compositions of the general formulas (AxBiyB209), in which A = Bi, Ba, Sr, Ca, Pb, Na, K, Cd or their mixtures, B = Ti, Nb; Ta or their mixtures and n is an integer and where the sum of x + y equals 3, existing mass in the usual manner mixed, deformed and fired. Publications Considered: British Patent Nos. 583 494, 574 577; Journal of the American Ceramic Society; 1952, Pp. 207-214 (Niobate and Tantalate Dielectrics).