DE827615C - Improvement of ceramic dielectric material - Google Patents

Description

The invention relates to ceramic bodies which are suitable for use as dielectric There are material in capacitors and other objects. These bodies exist for the most part Part made of barium titanate, which can make up 90% or more of the body, for example. Such bodies are primarily of importance because of the high achievable value of themselves in them specific dielectric constant.

It has been found that bodies of this type, which have a relatively high dielectric constant of 500 and above, do not have a sufficiently high insulation resistance for many applications, especially when they are subjected to elevated temperatures or high electrical stress. Their insulation resistance decreases with increasing temperature. If, therefore, the insulation resistance is already low at room temperature, the rise in temperature when the voltage is applied will be relatively high and lead to a further decrease in insulation resistance. The effect increases, and a point can be reached at which the insulation resistance drops so sharply with increasing temperature that the material experiences thermal destruction. This property of the material often leads to a short service life for the device. So z. B. a typical body consisting essentially of barium titanate have a dielectric constant of about 1200, but also have a specific volume resistance at room temperature of between ¹⁰ and ^{10 12} ohms per cubic centimeter, which ^{falls to 10 9} to ^{10 11} ohms per cubic centimeter at 100 °. This fact seriously affects the general applicability of these bodies.

It has now been found that the addition of certain fluorides to these bodies even in very small amounts, e.g. B. ο, ι% upwards, has the effect of increasing the specific resistance to a remarkable degree. These fluorides are the fluorides of metals of the second group of the periodic table, namely beryllium, magnesium, calcium, zinc, strontium, cadmium and barium, as well as those of nickel, iron, sodium and lead and the mixed fluoride of sodium and aluminum of the formula Na ₃ AlF ₆ .

The fluorides of the metals of the second group are mentioned in British patent specification 586 802 on barium titanate bodies in amounts of 0.5 to 10% for the purpose of improving certain properties, in particular the resistance to thermal shock. No mention is made of the effect of the additive on the specific resistance.

The new article based on the invention is a body made of ceramic material, in which barium titanate is the predominant component and in which an addition of a fluoride of a metal of the second group in amounts of 0.1 percent by weight, based on the total weight of the body up to one Amount which is distinguishably less than 0.5% or a fluoride of nickel, iron, sodium and the like or a mixed fluoride of sodium and aluminum of the formula Na ₃ AlF ₆ in the amount of 0.1 to 10% the total weight of the body is present. The term distinguishable less than 0,5% as used here means not more than 0,48 ° / o.

The bodies can be mixed and shaped in a known manner and heat treated in the usual manner. The latter consists in placing a large number of the bodies, possibly on suitable supports, in a furnace, then closing the furnace, gradually increasing the temperature to the sintering range, i.e. about 1100 to 1350 ^° , and then raising the material to a relatively low one Temperature is allowed to cool,

Kör
by composition Specific resistance
Ohms / cc
25 ⁰ I 70 ⁰ - nd
100 ° Dielek
tricity
constant Performance
factor at
ι Mc / s Performance
factor at
iKc / s T t A. Barium titanate .. 99
Nickel fluoride .... 1
Boric acid 1
Bentonite 1 1.06 · io ¹³ 2.89. io ¹² I910 2.2% - 1250 25th B. Barium titanate .. 98.5
Nickel fluoride 1.5
Boric acid 1
Bentonite 1 1.35 · io ¹³ 9.2 io ¹³ 2.94 · io ¹² 2070 0.4% 1350 15th C. Barium titanate. .99
Sodium fluoride .. 1
Boric acid 1
Bentonite 1 9.2 io ¹³ 1000 2.5% II50 T5

before the oven is opened. In such a treatment, the rise in temperature can be up to the peak value over a period of 10 to i2Stunden or longer extend, and cooling from the peak temperature to 900 ⁰ down can take place within 2 to 3 hours or more. However, in order to take full advantage of the desired result, it is preferred to prep the bodies using an improved method. This method is characterized in that the temperature to the sintering temperature or a higher temperature increases the body by 900 ° and then its temperature is lowered to 900 ^0, where the total time above 900 ⁰ 3 hours in general, and for 1 hour at bodies having a wall thickness of less than 0.5 mm. The essential fact here is the reduction of the cooling time in the range from the peak temperature to 900 ⁰ down to a small value, e.g. B. to about 15 minutes. Reducing the time it takes to go from normal temperature to peak can also help improve results. A time of 15 minutes for the increase leads to satisfactory results and can advantageously be reduced from time to time.

An important factor in obtaining full benefit of the invention is relative short time which is spent at or near the sintering temperature.

Examples of different bodies containing metal fluorides are still used to explain the Invention given.

The bodies were obtained by pressing disks from an intimate mixture of the ingredients at a pressure of 422 kg / cm -1. The size of the disc was approximately 1.25 cm in diameter and 0.3 cm in thickness. They were quickly fired in 8 minutes by bringing each body to the maximum temperature range, which was T ⁰ , holding it in that position for t minutes, then immediately removed from the furnace and allowed to air cool.

Kör
by composition Specific resistance
Ohms / cc
25 ° I 70 ° 1.2 IO ¹³ nd
100 ° Dielek
tricity
constant Performance
factor at
ι Mc / s Performance
factor at
ι Kc / s T t D. Barium titanate. 95
Sodium aluminum
fluoride 5
Boric acid 1
Bentonite 1 9.6 io ¹³ 1.1 · IO ¹³ 1.9% IIOO 15th E. Barium titanate .. 99
Lead fluoride 1
Boric acid 1
Bentonite 1 9.0 io ¹³ 850 2.9% II50 15th F. Barium titanate .. 99
Iron fluoride 1
Boric acid 1
Bentonite 1 6.5 io ¹² 2.1 · IO ¹² 1200 2% 1200 15th

Silver-plated electrodes were placed in the bodies by known methods.

Dielectric constant and power factor were measured at a temperature of 25 °.

Claims (5)

Patent claims: i. Ceramic body with barium titanate as the predominant component, characterized in that it contains one of the following metal fluorides in the specified amount: fluorides of nickel, iron, sodium, lead or mixed fluoride of sodium and aluminum of the formula Na ₃ AlF ₆ in the amount of ο, ι to ι ο ° / o of the total weight of the body or fluorides of the metals of the second group of the Periodic Table in the amount of 0.1 to 0.48 ⁰ A of the total weight of the body. 2. Ceramic body according to claim 1, characterized in that it contains 90 or more percent by weight of barium titanate. 3. Process for the production of a ceramic body according to the preceding Proverbs, characterized in that substances are heated together, which to the greater Part of barium titanate and an addition of a fluoride of a metal of the second group of Periodic table in the amount of 0.1 to 0.48 percent by weight of the total weight of the Body exist. 4. A method for producing a ceramic body according to claims 1 and 2, characterized in that substances are heated together, which for the most part from Barium titanate and an addition of a fluoride of nickel or iron in the amount of 0.1 to Composed of 10% of the total weight of the body. 5. A method for producing a ceramic body according to claims 1 and 2, characterized in that substances are heated together, the greater part of which consists of barium titanate and an addition of a fluoride of sodium, lead or the mixed fluoride of sodium and aluminum of the formula Na ₃ AlF ₆ consist in amounts of 0.1 to 10% of the total weight of the body. ' φ 2718 12.