DE3103878C2 - Voltage-dependent resistor based on zinc oxide - Google Patents

Abstract

The invention relates to a voltage-dependent resistor with a high n-value made from a sintered mass which consists essentially of zinc oxide and additive dopants and is characterized in that an oxygen-transferring substance is added to the sintered mass in such a way that zinc oxide decomposition is largely avoided during the sintering process. The oxygen-donating substance is mixed as homogeneously as possible with the sintered mass, so that the oxygen supply during the decomposition as a result of the heating process during sintering can satisfy as many zinc ions as possible, i.e., due to the great affinity between zinc and oxygen, as many lattice defects as possible can be occupied by oxygen , whereby the additive dopants are displaced and accumulate on the grain boundaries of the zinc oxide. While maintaining the C value, very high n values can be achieved without the need for an expensive process. The manufacturing-related spread of the C-values is reduced. Furthermore, the invention enables a less critical sintering process, i.e. the varistor property no longer depends critically on the adherence to certain sintering parameters.

Description

Ig (72/71)

The invention relates to a voltage-dependent zinc oxide-based resistor, mainly composed of Metal oxides are added to existing aggregates, according to the preamble of claim 1.

Such a voltage-dependent resistor is known from DE-AS 18 02 452.

Voltage-dependent resistors are advantageous due to their voltage-limiting effect to protect electrical devices and systems from overvoltages or current peaks in a wide variety of ways Voltage ranges used.

The resistance behavior can be related to the relationship

GQ "

describe, where / is the current flowing through the resistor, U is the voltage across the resistor and C is a constant corresponding to the voltage for a given current. The exponent π is a measure of where UX and i / 2 are the voltages at given currents 71 and / 2, respectively. In general they are high

to values of η are desired, then the deviation of the resistance from the purely ohmic behavior is large. The value of C depends on the application and must be dimensioned accordingly.

A number of non-linear resistors are known, in particular based on silicon carbide, titanium oxide, copper oxide or zinc oxide, the latter being able to provide very high values of η.

Such zinc oxide varistors represent a network of highly conductive areas and barrier layers, with voltages below C the barrier layers are in the blocked state and with voltages above the value C the majority of the barrier layers are conductive because their blocking voltage threshold is exceeded and thus the high-conducting areas bring about a low total resistance value.

There are several manufacturing processes for ZnO varistors became known, with essentially zinc oxide as the main component with various additives be ground, mixed, pressed, sintered and with one or more of these operations under Repeated several times under certain circumstances. The finished varistor is present when, for example, on the plane-parallel lobed end faces electrodes are attached.

As already indicated above, the highest possible values of η and thus a large non-linearity are desired. The aim of known manufacturing processes is to achieve a further increase in the n values, but this is generally only possible with a simultaneous increase in the C values (DE-OS 24 50 180). According to today's view, the non-linearity of a varistor depends primarily on the nature of the grain boundaries. It is of great importance that the metal oxides added to the zinc oxide concentrate in the marginal areas of the zinc oxide grain as a barrier layer that is as homogeneous as possible.

The invention was based on the object of specifying a varistor based on zinc oxide of the above type, which is characterized by particularly high n values with little scattering of a low C value and by a less critical underpass in its manufacture.

The solution erf.ilgt with those characterized in claim 1 Features. Advantageous refinements result from the subclaims.

DE-OS 29 10 841 discloses a voltage-dependent resistor material and a method for it Manufacture has become known in which a very high non-linearity factor π is also achieved without that the C-value is impaired or changed, in particular increased. This is achieved by that a largely homogeneous, uniformly thin, only minimally diffused into the peripheral areas of the zinc oxide and the barrier layer surrounding the entire zinc oxide grain can be realized.

From relevant chemistry textbooks such as Holleman, Textbook of Chemistry, First Part, Inorganic Chemistry, Gruyter, Berlin 1947, are

Compounds such as perborate, perchromate and permanganate known per se as a strong oxidizing agent. Their use in the context of the application however, it cannot be concluded from this.

The invention was based on the knowledge that defects in the zinc oxide lattice structure are the preferred location of the added dopants, the formation of these lattice defects being due to the decomposition of the zinc oxide can be returned during sintering, according to the following chemical reaction equation is described:

of about 150 "decomposed according to the reaction equation:

ZnO,

> 150 °

ZnO + -O ₂ .

Other advantages of using zinc peroxide as an oxygen donor are its non-explosive and non-explosive toxic properties.

ZnO

Even when sintering in an oxidizing atmosphere, even in a pure oxygen atmosphere, the zinc oxide can decompose cannot be avoided entirely.

The idea of the present invention was to add an oxygen-releasing substance to the material to be sintered, d. H. an oxygen dispenser to be added in such a way that the above-described decomposition of the zinc oxide during the sintering process can largely be avoided. The oxygen-donating substance is used in a favorable manner Mixed as homogeneously as possible with the mass to be sintered, so that the oxygen supply during the decomposition as a result of the heating process during sintering, as many zinc ions as possible can be satisfied, d. h "that due to Because of the great affinity between zinc and oxygen, as many lattice defects as possible are caused by oxygen kc.men are occupied, whereby the additive dopants are displaced and borrowed at the grain boundaries of the zinc oxide.

The varistor according to the invention has the advantage that, while maintaining the C value, very high n values can be achieved without a complex process is required. Furthermore, it has been shown that the manufacturing-related spread of the C-values even decreased will. It is also advantageous that the invention enables a less critical sintering process, d. This means that the varistor property is no longer critical Way depends on compliance with certain sintering parameters

In one embodiment of the invention, it is possible that the oxygen dispenser also provides a complete or almost complete combustion of the organic binder is achieved.

Oxygen-releasing substances within the meaning of the invention are substances that when exposed to heat or other physical forms of energy release oxygen in abundance. These are mostly made of fabrics the group of peroxides, chromates, permanganates, perborates or similarly acting compounds.

In a further embodiment of the invention, there is an extensive preservation or non-disruptive effect Change in the microstructure of the mass to be sintered favorably, so that the ions of the oxygen donor can be easily incorporated into the lattice structure of the zinc oxide.

In order to avoid the above-mentioned decomposition of zinc oxide into metallic zinc, the must during sintering resulting oxygen partial pressure must be correspondingly large.

Zinc peroxide has a very beneficial effect as an oxygen donor, which in particular takes advantage of the claims 5 and 6 yields, it being from a temperature

Claims (7)

Patent claims: 1. Voltage-dependent resistor with a high n-value made from a sintered mass that is essentially consists of zinc oxide and additive dopants, characterized in that the to sintering mass an oxygen-releasing substance has been added such that during the sintering process decomposition of the zinc oxide has largely been avoided 2. Voltage-dependent resistor according to claim 1, characterized in that the proportion of the oxygen-releasing substance has been 0.1 to about 90 mol% 3. Voltage-dependent resistor according to claim 1 or 2, characterized in that the to sintering mass contains an organic binder and that by the oxygen-releasing substance in addition the most complete possible combustion of the organic binder has been achieved 4. Voltage-dependent resistor according to one of the preceding claims, characterized in that that the oxygen-releasing substance of the group of peroxides, chromates, permanganates, who belonged to Perborate or similar groups 5. Voltage-dependent resistor according to one of the preceding claims, characterized in that that the oxygen-releasing substance contains ions through which the microstructure of the to be sintered Mass has remained largely unchanged or undisturbed S. Voltage-dependent resistor according to one of the preceding claims, characterized in that that the oxygen-releasing substance is in a favorable range below the sintering temperature has shown lying decomposition temperature. 7. Voltage-dependent resistor according to one of the preceding claims, characterized in that that the oxygen-releasing substance was zinc peroxide or zinc chromate. the non-linearity of the resistance and results from the relationship