DE2343142C3 - Non-porous ceramic dielectric and means for its preparation - Google Patents

Description

US-PS 23 98 088 discloses a capacitor in the form of a vitrified product in which TiO ₂ with a high dielectric constant and a negative temperature coefficient of the dielectric constant and a compound of a rare earth metal with a high dielectric constant and a low power factor and a positive temperature coefficient of the dielectric constant are embedded. With such dielectrics, dielectric constants of the order of magnitude of about 40 are generally achieved. US-PS 36 19 220 discloses a dielectric whose advantage is said to be a lower firing temperature and which contains titanates, zirconates and stannates of rare earth metals in a glass bond. Bismuth or calcium oxide, among others, can be present in the glassy flux phase. This dielectric is extremely sensitive to AH, whereby the capacity drops rapidly. In addition, this known dielectric has a relatively low discharge resistance. A similar dielectric is known from US Pat. No. 82,766 in the form of a glass ceramic, the ceramic phase of which contains oxides of magnesium, calcium and rare earth metals as titanates.

The glass phase is an oxide mixture of cadmium, bismuth, lead, aluminum and silicon. N 750 bodies made from such dielectrics have dielectric constants in the order of about 75, while these are probably only about 36 for other common products.

The object of the invention is to provide a dielectric which is characterized by a particularly high dielectric constant, a low and adjustable temperature coefficient of the dielectric constant, low dielectric losses and a high discharge resistance and which is dense. Starting from a ceramic dielectric based on rare earth metal titanates, the essentially non-porous, dense dielectric according to the invention has a dielectric constant of approximately 55 to 90, a temperature coefficient of the dielectric constant of approximately -100 to +100 · 10- ⁶ ZK, tan δ 0.001, in particular a maximum of 0.0003, a discharge resistance at 150 ⁰ C of £200; in particular >1000Ω·�R; and contains about 12 to 20 mol% neodymium oxide, about 12 to 20 mol% barium oxide, about 70 to 60 mol% titanium dioxide, about 1.5 to 5 mol% bismuth oxide, 0 to about 5 mol% zirconium oxide, 0 to about 5 mol% tin oxide, 0 to about 10 mol% calcium and/or strontium oxide and up to 5 mol% other rare earths, the sum of the oxides of titanium, tin and zirconium being about 60 to 70 mol% and the sum of barium, calcium and strontium being about 12 to 25 mol%. The proportion of other rare earths should preferably be < 10% of the neodymium oxide content.

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To produce the dielectric according to the invention, a suitable agent is one consisting essentially of about 12 to 20 mol% neodymium oxide, about 12 to 20 mol% barium oxide, about 70 to 60 mol% titanium dioxide, about 1.5 to 5 mol% bismuth oxide, 0 to about 5 mol% zirconium oxide, 0 to about 5 mol% tin oxide, 0 to about 10 mol% calcium and/or strontium oxide, whereby the proportion of rare earths can be up to about 5 mol% and the sum of the oxides of titanium, tin and zirconium can be about 60 to 70 mol% and that of barium, calcium and strontium can be about 12 to 25 mol%. It is useful if the oxides of barium, calcium, strontium and bismuth are present in the form of their titanates, zirconates or stannates. The seodymium can be pre-reacted with the titanium dioxide, in which case the ratio of the two components is approximately 68:32 to 52:48.

The firing temperature for the dielectric according to the invention is between about 1220 and 1300 ^° C and can vary over wider ranges compared with known dielectrics.

As mentioned, the dielectric according to the invention is characterized by a low temperature coefficient of the dielectric constant within the NPQ range, which is in particular between -30 and +30 × 10- ⁶ /K, so that capacitors for circuits with high temperature stability can be produced therewith.

Another advantage of the dielectric according to the invention is the firing temperature required during its production. Many ceramic materials are very sensitive in this respect and must be fired in very narrow temperature ranges in order to achieve satisfactory results.

to achieve satisfactory dielectric properties. However, this increases production costs, in particular due to high levels of waste. The agents according to the invention are relatively insensitive to fluctuations in firing temperatures, which can be 40 K or even 80 K, which means that high-quality capacitors can be produced very economically.

There are various possibilities for producing the dielectric according to the invention. The components can be wet ground for several hours, particularly in a ball mill. The grinding time depends on the water content of the material to be ground, the size of the balls, the filling of the mill, etc.

After grinding, the discharged slip is dried in the usual way, e.g. on trays or belts or by spray drying. The dry material is granulated if necessary in order to break up large pieces. If it has been dried on a tray, a dry slip may be present which can be divided by pressing it through a coarse sieve, such as a sieve with a mesh size of about 1.65 mm.

Now it is fired at a temperature between about 1000 and 1200 ⁰ C in an oxidizing atmosphere so that all components are present as oxides.

These oxidic masses can be combined substances of two or more oxides, such as barium titanate, barium zirconate, barium stannium, bismuth zirconate, calcium irconate and the titanates of bismuth, calcium or strontium. The oxides of the alkaline earths (barium, calcium and strontium) are reactive with water and carbon dioxide. Accordingly, they are used for the masses according to the invention in pre-reacted form as double oxides, such as the titanates, zirconates or stannates.

!In the mixture to be burned, substances that are not oxides can also be used as starting materials, but which turn into oxides when burned. The neodymium oxide content can be used in the form of neodymium carbonate or oxalate or as oxide hydrate or hydroxide.

Another possibility is to first pre-mix the neodymium oxide with titanium oxide in a weight ratio of about 68:32 to 52:48 and homogenize it together with water in a ball mill. This slip is then dried and any lumps that may have formed are broken up. It is then fired again at about 1000 to 1200 ⁰ C in an oxidizing atmosphere.

After firing the mixture of neodymium oxide and titanium oxide, the fired material is mixed together with the other components in a ball mill, the slip is dried and the cake is granulated if necessary. This is followed by firing at around 1000 to 1200° C in an oxidizing atmosphere. Firing is not necessary if all components are already in a pre-reacted state. The neodymium oxide can - as mentioned above - be in a pre-reacted state. Barium titanate, barium 60-um zirconate, calcium titanate, calcium stannate, bismuth titanate and bismuth zirconate are produced in a similar way. If titanates, stannates and/or zirconates are also used, the last firing stage can be omitted, but in some cases it is preferable to fire again at the end.

A significant advantage of the dielectrics according to the invention is that they are essentially non-porous. Porosity is generally determined using the ink test. A dielectric is immersed in ink for 30 seconds, then removed, quickly rinsed with water and the surfaces wiped with a dry cloth. If the material is essentially non-porous, no visible ink discoloration is seen on the surface. Such a material is rated 1. If there is slight surface discoloration, this is rated 2. If a certain penetration of ink into the body is observed and the surface is also colored, this corresponds to a value of 3. If a greater penetration into the body is observed, the material is given a value of 4. Under the above test conditions, the dielectrics according to the invention almost always give a value of 1, which proves the essentially pore-free structure. The ink used for this test is not critical; you can use any aqueous ink or dye.

When producing the dielectrics according to the invention, the proportions of the components in the mixture can be chosen so that the corresponding values for the temperature coefficients of the dielectric constant are obtained. Titanium oxide. Neodymium oxide and bismuth oxide shift the temperature coefficient towards the positive side; calcium, strontium, zirconium and zinc oxide to the negative side. Bismuth titanate and barium titanate are almost neutral: by increasing the components shifting to the negative side, a dielectric with a more negative temperature coefficient of the dielectric constant is obtained. Conversely, increasing the positively shifting components leads to more positive values of the temperature coefficients for the dielectric constant.

Various substances, such as mineralizers or electrical modifiers, can be present in small amounts of about 0.1 to 1% by weight, such as silicon dioxide, cobalt oxide, manganese dioxide, zinc oxide, zinc titanate, niobium oxide and tantalum oxide.

The invention is further explained using the following examples. All parts and percentages are by weight unless otherwise stated.

Examples

The starting materials were ground in a ball mill, the mill output dried, granulated and fired in an oxidizing atmosphere at 1150 ⁰ C. In Examples 1 to 7, neodymium and titanium oxide were ground in the ball mill, dried, granulated and pre-reacted at 1150 ° C in an oxidizing atmosphere and the neodymium titanate was then ground together with the other components, dried, granulated and fired at 1150 ⁰ C in an oxidizing atmosphere. In Examples 7 to 13, the neodymium and titanium oxide were not pre-reacted in the above sense, but were added separately to the other components, then dried, granulated and fired at 1150 ⁰ C in an oxidizing atmosphere.

The ceramic materials according to Table I were then processed into ceramic dielectric bodies. This was done by grinding the ceramic material in a ball mill for about 20 hours with an acrylate binder, namely for 100 parts of ceramic powder, about 50 parts of plastic and an additional 50 to 100 parts of chlorinated hydrocarbon as

solvent and about 0.4 to 0.5 parts of plasticizer were added. After grinding, the mill contents were poured onto a glass plate and, after adjusting the thickness of the applied mass to a layer thickness of about 76 μιη, was allowed to dry in the air at room temperature. This layer was cut into rectangular pieces of about 76 x 25 mm and a stack of these pieces was pressed for a short time (about 10 to 20 s) under a pressure of about 4.5 μm, so that a body with a thickness of about 0.66 to 0.71 mm was obtained. This material was divided into 12.7 mm cubes and these were fired on zirconium oxide support plates in an electric tunnel furnace. The furnace worked with a 7-hour program, ie the passage time through the furnace was about 7 hours, with the material remaining for about 1 hour at the firing temperatures as given in the following table.
The opposite sides of the bodies, which had shrunk by about 25%, were coated with a silver conductive lacquer, which was dried and kept at 815° C for about 15 minutes to make the lacquer flow. The two conductive layers were then contacted and the electrical measurements for the

&iacgr;&ogr; Determination of the dielectric properties of the dielectric. The production of the dielectric itself is the usual procedure and not the subject of the present invention. The new dielectrics can also be formed in other ways.

Table i

At- _{BaTiO3 BaZrO3 BiTiO3 BiZrO3 CaTiO3 CaSnO3} CaZrC ) _{3 Nd2O3 :} TiO2 _{Nd2O3r ​} tan ; sp _f e! "! &bull;l *> &bull;&igr; *)■ &bull;1 TiOj "I &diams;i -iir* 58:42

20 20 25 20 22 22 24 24 24 24 24 14 20

20 20 15 12

10 10 10 10

10 10

*) % by weight

Firing temperature: Example 1 1240 C" Example 2-13 1280 C

10

Burning time: about 1 hour

60
60
60
64
66
66
60

25.2

25.2

25.2

26.9

27.7

27.7

25.2

25

25

26.5

25

32

25

34.8

34.8

34.8

37.1

38.3

38.3

34.8

33.5

87 87 81 79 75 78 69 67 66 78 70 55 80

-17 -17 -07 -08 -19 -33 -66 + + -27 +24 +49 +

!300

1800

3000

6000

4800

2800

17000

30000

3800

&Iacgr;200

5000

1800

Table II

Example BaO') CaO') Bi ₂ O ₃ *) Nd ₂ O ₃ ') TiO ₂ *) SnO ₃ *) ZrO ₂ *) BaO/CaO*) TiO ₂ /SnO ₂ /ZrO ₂ *)

9
10
11
12
13
*) Mol-%

12.5 12.5 15.2 14.2 15.6 13.0 17.6 17.7 14.1 13.7 13.7 8.6 15.6

2.7

4.5 4.5 5.9

4.7 4.7 3.4 2.7 1.8 1.8 2.3 2.2 2.2 2.1 2.1 2.3 1.8

15.1 15.1 14.6 15.6 16.4 15.7 14.4 14.6 14.1 13.2 13.9 18.7 16.4

67.7 66.7 66.8 65.4 64.2 64.1 62.7 62.5 60.6 62.0 61.9 70.4 64.2 2.7

2,J 15.7 2.0 3.0 18.6 3.0 18.2 4.5 19.6 4.5 3.0

2.0 23.05

67.5 66.2 66.8 65.7 65.5 65.1 66.5 64.4

66.2

The neodymium oxide used in the examples was a commercially available product with a purity of 95% (the remainder being other rare earths). The bismuth zirconate mentioned in the table was a double oxide of about 72 to 74 wt.% bismuth oxide and 27 to 25 wt.% titanium dioxide. The bismuth zirconate was a double oxide of about 72 wt.% bismuth oxide and 27 wt.% zirconium dioxide, containing about 1 wt.% silicon dioxide. In the pre-reacted neodymium oxide/titanium dioxide, the

Weight ratio 58:42. As mentioned, in Examples 1 to 7 neodymium oxide and titanium dioxide were pre-reacted; the values given in the table are the total values for titanium dioxide and neodymium oxide in the pre-reacted state within the batch. In Examples 8 to 13 the two oxides were present separately in the batch.

The starting materials mentioned in the examples were commercially available products. The barium titanate contained 33.5 to 33.9 wt.% TiO ₂ and 64.2 to 63.8 wt.% BaO as well as small amounts of SiOj, Al ₂ Oj, SrO and Na ₂ O. The barium zirconate contained 41 to about 44 wt.% ZrO ₂ , 55 to 53 wt.% BaO, about 2 to 3 wt.% SiO ₂ and small amounts of TiO ₂ and Al 2 O 3 . The calcium stannate contained 64.8% tin oxide and 23.91% calcium oxide, the calcium titanate 39 to 41% CaO and 59 to 55% TiO ₂ as well as small amounts of SiO ₂ and Al ₂ O ₃ . Calcium zirconate contained 65 to 68.5% ZrO ₂ , about 31 to 28% CaO and about 3 to 4% SiO ₂ with small amounts of TiO ₂ UnUAl ₂ Oi

The &egr; values are the dielectric constants of the ceramic bodies at 25°C. At the same temperature, the values for the Loss factor determined at 1 kHz. The information on the temperature coefficient of the dielectric constant applies to the temperature range from 25 to 125° C.

It is clear from Table 1 that the ceramic materials according to the invention have ε values in the order of about 50 to 90 and at the same time show relatively low temperature coefficients of the dielectric constant. The higher

ε values are achieved without compromising the stability of the dielectric constant at varying temperatures. The dielectric losses are low. The values for the leakage resistance are obtained at 500 V for 2 minutes from the Resistance of the dielectric. The capacitance and resistance of the dielectric were measured at 125° C. The product of capacitance and resistance is then the conduction resistance.

It is noteworthy that the insulation strength of the

dielectric and provides a basis for comparing capacitors of different sizes. High values show that the electrical losses of the dielectrics according to the invention are low, so that they can be used in capacitors with very high

capacities can be applied.

Table II shows that in the dielectrics according to the invention with outstanding properties, the proportions of metal oxides are in the above ranges. If Example 11 is repeated.

However, if strontium titanate is used instead of calcium titanate, you also get a dielectric with good electrical properties.

Claims (5)

Patent claims: 1. Dense, essentially non-porous ceramic dielectric based on rare earth metal titanates with a dielectric constant of about 55 to 90, a temperature coefficient of the dielectric constant of about -100 to +100 - 10- ⁶ /K', tano< 0.001, in particular a maximum of 0.0003. a discharge resistance at 150 ⁰ C of > 200, in particular Ω-F, characterized by a content of about 12 to 20 mol% neodymium oxide, about 12 to 20 mol% barium oxide, about 70 to 60 mol% titanium dioxide, about 1.5 to 5 mol% bismuth oxide, 0 to about 5 mol% zirconium oxide, 0 to about 5 mol% tin oxide, 0 to about 10 mol% calcium and/or strontium oxide and up to 5 mol% other rare earths, the sum of the oxides of titanium, tin and zirconium being about 60 to 70 mol% and of barium, calcium and strontium being about 12 to 25 mol% 2. Dielectric according to claim 1, characterized in that the proportion of other rare earths corresponds to < 10% of the neodymium oxide content. 3. Means for producing the dielectric according to claim I or 2, consisting essentially of about 12 to 20 mol% neodymium oxide, about 12 to 20 mol% barium oxide, about 70 to 60 mol% titanium dioxide, about 1.5 to 5 mol% bismuth oxide, 0 to about 5 mol% zirconium oxide, 0 to about 5 mol% tin oxide, 0 to about 10 mol% calcium and/or strontium oxide, where the proportion of rare earths can be up to about 5 mol% and the sum of the oxides of titanium, tin and zirconium is between about 60 and 70 mol% and of barium, calcium and strontium is between about 12 and 25 mol%. 4. Center! according to claim 3, characterized in that oxides of barium, calcium, strontium and bismuth are present in the form of their titanates, zirconates or stannates. 5. Agent according to claim 3 or 4, characterized in that the neodymium oxide is present as a pre-reacted mass with titanium dioxide at a weight ratio of neodymium oxide:titanium dioxide of about 68:32 to 52:48.