DE1116742B - Ferroelectric ceramic mixture for an electromechanical transducer element - Google Patents

Description

The invention relates to ferroelectric ceramics, the polycrystalline aggregates of certain Represent the texture and are fired to ceramic maturity and then polarized or are polarizable so that they are given electromechanical transducer or transducer properties that are similar to the piezoelectric effect. The invention particularly includes the physical mixtures of raw ingredients, the precursors of such ceramic materials are the reaction product of such mixtures and those made from the matured ceramic electromechanical transducers or converters. The term "ceramic mixtures" is intended for everyone physical mixtures which have not reacted, as well as the mixtures which have reacted with one another and include the mature ceramic that is the final product. The term "ceramic materials" is used especially for fired or matured ceramics.

The ceramic compositions or mixtures according to the invention have as the main component Compilations of the binary system lead titanate - lead zirconate or lead titanate - lead tannate or of the ternary system lead titanate - lead zirconate - lead tannate.

Because of their low cost and their great durability under adverse atmospheric conditions, the ferroelectric ceramics have gained great importance as a substitute for crystals in the field of transducers in the generation, measurement and / or perception of sounds, shock vibrations, pressures, etc. Promising ceramics for this purpose include lead zirconate titanate, a polycrystalline material that is actually composed of lead zirconate (PbZrO ₃ ) and lead titanate (TbTiO ₃ ) in solid solution. In certain areas of the composition based on the mole percent of its constituents, lead zirconate titanate exhibits extremely desirable electrical and mechanical properties, particularly strong electromechanical coupling when electrostatically polarized (U.S. Patent 2,708,244).

Substances of the ternary system lead zirconate — lead titanate — lead tannate (Pb Zr O., - Pb Ti O ₃ -Pb Sn O) (USA. Patent 2,849,404) are similar in structure and properties to lead zirconate titanate. Within certain ranges, bodies made from these systems also show remarkable ferroelectric properties, by means of which they can be used in the same general fields of application as the lead zirconate-ferroelectric ceramic mixture
for an electromechanical transducer element

Applicant:

Brush Crystal Company Limited,
Southampton (UK)

Representative: Dr.-Ing. W. Reichel, patent attorney,
Frankfurt / M. 1, Parkstrasse 13th

Claimed priority:
Great Britain of September 15, 1958 (No. 29 445/58)

John Anthony Sugden, Horsell, Woking, Surrey

(Great Britain),
has been named as the inventor

Titanate ceramics can be used. A large number of such mixtures of the lead titanate-lead zirconate-lead stannate system are in the »National Bureau of Standards Report No. 3684 «(Jäffe, Roth and Marzullo - Report No. 9 - October 1, 1954) under the title »Improvements in Piezoelectric Ceramics ”and more fully in the“ Research Paper 2626 ”by the same authors under the Title »Properties of Piezoelectric Ceramics in the Solid-Solution Series Lead Titanate-Lead Zirconate-Lead Oxide: Tin Oxide and Lead Titanate-Lead Hafnate "of the" Journal of Research of the National Bureau of Standards ", vol. 55, no. 5, November 1955, pp. 239-254.

Pre-polarized ceramics for piezoelectric or electrostrictive applications are the subject a comparatively recent development; albeit the intensive study in this area has advanced by researchers towards the manufacture of fabrics that are commercially acceptable are, nonetheless, many problems need to be resolved, to varying degrees with all of them known ferroelectric ceramics are linked before the full capabilities of this Substances can be recognized.

A disadvantage of the ceramic transducer materials compared to the inherently piezoelectric ones Crystals consists in the fact that they have one polarization

109 7S8 / 270

3 4

must be subjected to electromechanical and the proportion of closed porosity (i.e. the

to make sensitive. As in the following, more precisely, pores that are formed by the surrounding material against a

is described, is used for polarization ferroelek- connection with the external environment of the body

Irish ceramics have a relatively strong electrostatic seal, immediately adjacent bubbles)

cal field applied to the ceramic body. If 5 are kept as low as possible. The density of the

it burned in this general approach ceramic naturally takes when the

is a relatively simple process as it is made from porosity decreases; an ideal ceramic body

for various reasons in its practical application naturally approaches the density as closely as possible

application is often extremely difficult to carry out. One of a perfect single crystal made of the same material

For these reasons is simply the property io rial.

of many useful ferroelectric ceramics, besides their contribution to low density and that they have a large coercive force and that is one of the undesirable effects associated therewith accordingly difficult to polarize on their own the open porosity furthermore extremely undesirable, are. Examples of ceramic substances with great because in their presence vapors, liquids and Coercive force are lead zirconate titanate mixtures, with 15 solid substances that can be absorbed, which is disadvantageous which part of the lead by strontium and / or the electrical properties, e.g. B. the specific Calcium is replaced. Resistance and dielectric strength in many areas of application these ceramics are very useful and flow. In addition, when applying a have the advantage of a high dielectric con-electrode coating made of metal, e.g. B. by standing up, ao steam or brush, the electrode metal in

In order to be able to polarize many ceramic materials that penetrate the open pores of the ceramic,
one makes use of stronger polarizing fields It is the aim of the invention to at least some

or elevated temperatures; this way out can be overcome until the difficulties outlined above

Promising to a certain extent, but the den. It consists in the creation of ferroelectric

Applicable field temperature conditions are 25 see ceramic mixtures, which by a slight-

limited by the fact that the specific polarizability than the usual mixtures

Resistance of ceramics is inversely with that of generally comparable quality

Temperature changes; consequently a short term arises and which also go up to a higher one

Conclusion and / or a breakthrough in the material before dimensions can be polarized and accordingly

the desired degree of polarization has been achieved. 30 have higher piezoelectric properties. These

Likewise, there is a strong opposite blends are also better sinterable and under

the dielectric strength depends on the thickness, flammable under moderate conditions and provide a

so that thick sections relatively very difficult to mature ceramics, the porosity of which is low, so that

are polarizing. The problem of polarization becomes its density that of the single crystal of the material

therefore particularly acute when approaching a massive 35.

Pieces busy and the field a very considerable- According to the invention a mixture of

borrowed material thickness, e.g. B. 1 inch (2.5 cm) or a base material (at least 95 percent by weight),

more, must enforce. such a binary solution made of lead titanate

In all of these previously mentioned cases, a polar (PbTiO ₃ ) and lead zirconate (PbZrO ₃ ) or lead is sometimes only with considerable difficulty - titanate and lead tannate (PbSnO ₃ ) or such, and then only up to a relatively low ternary Solution of the three substances mentioned represents part of the overall performance of the material that the composition within the rectangle is feasible. ABCD of the triangular mixture diagram (Fig. 3)

As a result of these difficulties, for some it is in the molar ratios of these three substances in which
Ceramic materials previously used polarization 45

process with an undesirably high amount of the points by 35% PbTiO ₃ , 65% PbSnO ₃ ,

Reject and in many cases with an insufficient point due to 55% PbTiO ₃ , 45% PbSn O _r

borrowed polarization connected. , _, _ . _{,, Λ} , ~ _m . ', ", ™",'

Another important goal in making the point C by 60% Pb Ti O ₃ , and 40 0/0PbZrO ₃

ferroelectric ceramic converter is the 50 derPunktD by 10% PbTiO ₃ , 90% PbZrO ₃
a high density, i.e. densities that differ

as far as possible to the density of single crystals, with 25 atomic percent of each approximate from the material. Lead through at least one alkaline earth metal, namely

The density of a ferroelectric ceramic is replaced by barium, calcium and / or strontium Body is intimate with many physical and electrical 55 can be, and from 0.01 to 1.0 percent by weight tric properties. In addition to the more (based on oxide) of an additive iron, or less clear relationship between density and nickel and / or cobalt after firing to ceramic mechanical Strengths are of a high density mix maturity and according to their polarization as electro also greater dielectric strength, a mechanical transducer element is used. Further higher dielectric constant, a lower mecha- 60 the mixture can except the 95 weight percent niche damping and better electromechanical of the base material up to 5 percent by weight tantalum, Coupling assigned. Niobium or a rare earth including yttrium,

In order to produce polycrystalline, ferroelek- but with the exception of cerium and / or up to about Irish ceramics have a greater density - 1.5 percent by weight chromium, molybdenum, tungsten aim, it is essential that when sintering during 65 or uranium in oxide form (everything on an oxide basis of the firing process contain any “open porosity” (i.e. calculated).

Pores into which liquids or vapor from the other features of the invention are in dispose Surface from penetration) eliminates binding described with the drawings.

Fig. 1 shows a perspective side view of an electromechanical transducer according to the invention,

Fig. 2 is a side view of the transducer shown in Fig. 1,

Fig. 3 is a triangular mixture diagram: iin the at substances used to carry out the invention and

FIG. 4 is a diagram showing the better porosity of the ferroelectric ceramic materials according to FIG of the invention.

In Figs. 1 and 2, an electromechanical transducer 10 can be seen, the one as an active element preferably disk-shaped body 12 made of a piezoelectric ceramic material according to FIG Invention contains.

The body 12 is electrostatically polarized and has two electrodes 14 and 16 on both of it Sides. On the electrodes 14 and 16 lead wires 20 and 22 are attached with a soldering point, so that the converter can be connected to an electrical circuit. As is known an electromechanical converter is used to convert supplied electrical energy into mechanical energy to convert, and vice versa. Therefore, when the ceramic body is subjected to mechanical stresses is subjected, an electrical voltage is generated by the resulting internal stresses the lines 20 and 22 generated. Conversely, a voltage applied to the lines creates a mechanical deformation of the ceramic body 12. The term "electromechanical converter" is in Used in an extremely general sense and includes piezoelectric filters, frequency control devices and the like a; The subject of the invention can also be used in many other areas of application, on which materials with dielectric, piezoelectric and / or electrostrictive properties are needed.

As already mentioned, the invention is applicable to ferroelectric ceramic mixtures which have become known as lead zirconate titanate (US Pat. No. 2,708,244). These substances have a perovskite or pseusocubic lattice structure, are ferroelectric at a certain molar ratio between lead zirconate and lead titanate (PbZrO _{: J} : PbTiO ₃ ) and show a remarkably strong electromechanical coupling when they are electrostatically polarized. This general category includes chemical modifications and substances that can be considered derived from lead zirconate titanate. With these substances, some or all of the lead zirconate can be replaced by lead tannate. In connection with this, it should be emphasized that there are some uncertainties and differences of opinion regarding the stability and the separate existence of the lead annate; Lead tannate can be regarded as lead oxide (PbO) and tin oxide (SnO ₂ ) in a stoichiometric ratio which _{corresponds to the empirical formula PbSnO 3} (with a molar ratio of 1: 1).

It should be noted that the proportions by weight of the ingredients mentioned below are based on of their respective oxides are calculated.

The basic mixes fall into three categories:

1. in those who add to the binary system lead zirconate-lead titanate belong,

2. in those who lead to the binary system— Count lead titanate, and

3. in those belonging to the ternary system lead zirconate - lead tannate - lead belong to titanate.

The terms "binary" and "ternary" are in Used in conjunction with the base mixes without regard to any secondary components. As is well known to those skilled in the art, hafnium is found as an impurity in zirconium changing amount; In connection with the invention, hafnium can be used as a substantial equivalent of zirconium be considered; the presence of hafnium either as an impurity or as a substitute for Zirconium is allowed. However, because of the high cost of the hafnium compared to the use of zirconium Making hafnium uneconomical in the production of mixtures remains the possible Presence of hafnium not taken into account in the present description. Although examples of Mixtures with hafnium are given herein, it should be emphasized that several rare earths because of their rarity and relatively high cost, they are not economically viable compared to other elements are of interest if they are also fully useful from a technical point of view.

All possible basic compositions or mixtures falling into the three previously mentioned Systems falling are represented by a triangular diagram shown in FIG.

However, all of the mixtures covered by the diagram are not ferroelectric as a whole; many are only electromechanically active to a limited extent. However, the base mixtures according to the invention are those showing a strong piezoelectric sensitivity. For reasons of expediency, the planar coupling k ″ (known as radial coupling k _r and disk _{coupling k dlsc} ) of the pre-polarized disks to be tested is chosen as a measure of the piezoelectric activity. All polarized

and tested base mixtures within the horizontally hatched area of FIG. 3, which is bounded by a square ABCD , show a radial coupling coefficient of at least 0.10. The area bordered by the square ABCD contains binary, solid lead zirconate-lead titanate solutions which lie on the line DC , on which the molar ratio (Pb Zr O ₃ : Pb Ti O ₃ ) of the end components is from 90:10 to 40:60 changes. Among these baseline mixtures, those lying between points H and G have significantly higher radial coupling, with the highest coupling occurring when the molar ratio of Pb Zr O ₃ : Pb Ti O _{3 is} about 53:47 or 54: 46 is.

The basic binary compositions on line AB (PbSnO ₃ : PbTiO ₃ from 65:35 to 45:55) of the diagram of FIG. 3 are similar in structure to those on line DC , but characterized by a generally lower radial coupling, where the best coupling occurs in the mixtures between points E and F, that is to say when the molar ratio Pb Sn O ₃ : Pb Ti O _{3 is} in the range from 60:40 to 50:50.

In the ternary base mixtures within the ABCD range , inclusion of BJeistannate (PbSnO ₃ ) as a replacement for part of the lead zirconate (PbZrO ₃ ) causes a progressive lowering of the Curie temperature, but the mixtures retain a relatively strong radial coupling, especially in

to the area of the diagram that is formed by the square EFGH .

As previously mentioned, the base mixes can be modified as desired by part of the lead is replaced by barium, calcium and / or strontium. This atomic based replacement is up to a maximum amount of 25% Barium, calcium and / or strontium are permitted, with the range from 5 to 15% being preferred. the The main effect of this replacement is that the dielectric constant and the coercive force be enlarged.

Except for the replacement or in place of the replacement of lead with barium, calcium and / or strontium The basic mixture can be carried out in the hatched areas of the mixture diagram in FIG. 3 Addition of small amounts of niobium, tantalum and / or certain rare earths modified in oxidic form will. All rare earths including yttrium but with the exception of cerium work satisfactorily; the amount of additive including tantalum and / or niobium can be up to a maximum value of 5 weight percent calculated on the oxide basis.

Another permissible modification of the base materials is the addition of at least one element from group VIB of the Periodic Table in an amount that corresponds to an addition of up to about 1.5 percent by weight of chromium oxide (Cr ₂ O) on a molar basis.

The basic compounds enclosed by the square ABCD of the diagram of FIG. 3 and, as desired, by replacing the lead with barium, calcium and / or strontium and / or by adding niobium, tantalum, yttrium and / or other rare ends other than cerium and / or modified by the addition of Group VIB elements are further improved according to the invention by the inclusion of significant amounts of at least one additive or additional reagent, namely iron, nickel or cobalt.

These compilations can be made according to various ceramic processes known as such are to be made.

In a preferred method for preparing the base mixes, lead oxide (PbO), zirconium oxide (ZrO ₂ ) and titanium dioxide (TiO ₂ ) are used in combination in the correct proportions in a relatively high degree of purity (e.g. chemically pure). (In some cases a slight excess of lead oxide can be used to make up for the additives.) If barium, strontium and / or calcium are to be introduced, these are added in the form of an appropriate reactive ingredient. The carbonates of these alkaline earth metals, which are commercially available relatively cheaply in the appropriate degree of purity, are preferred. When these mixtures react, the carbonates give off carbon dioxide CO ₂ and reduce the oxides concerned. The optional additions of niobium, tantalum, elements of group VIB (Cr, Mo, W, U) and / or the rare earths, more appropriately their respective oxides, can be introduced into the mixture at this point in time. The combined ingredients are then wet or dry milled to achieve thorough mixing and particle size reduction.

After grinding, the mixture is either fluffed or sintered into desired shapes brought to a temperature of about 850 ° C for approximately 2 hours to a prereaction.

It is advisable to take into account the loss of lead during heating e.g. B. to regulate that the sintering is carried out in a shell containing a lead oxide emitting source (U.S. Patent 2 708 244). The specific sintering conditions depend of course on changeable factors such as the The size and shape of the mold, from and can be chosen according to the established ceramic process be, which is suitable in the particular case, the mixture as completely as possible to Reaction should be brought.

After the pre-sintering, the reacted material is allowed to cool; then it is pounded and grind it into small particles. After grinding, the pre-sintered mixture is ready to go brought into the desired shape and burned to maturity. Depending on the desired In the form of a mortar or slurry, the material can be made according to known methods can be pressed, cast or extruded.

The additives or reagents, i.e. iron (Fe), nickel (Ni) and / or cobalt (Co), can be introduced into the mixtures at an appropriate point in time before the final grinding, preferably at an early stage of the manufacturing process, so that they are thoroughly mixed and reduced in size the particle size is guaranteed. The additives can be in the form of their respective oxides or compounds, e.g. B. as carbonates, are used, which transform into oxide during firing. Calculated on the oxide basis, the minimum effective amount of the additional reagent is approximately 0.01% and the maximum amount is approximately 1.0% by weight. The oxides used in the preparation of the mixtures given below as an example are gray cobalt oxide (generally CoC), black nickel oxide (NiO) and red iron oxide (Fe ₂ O ₃ ).

The powder brought to reaction is brought into a suitable shape and up in a known manner Fired to ceramic maturity.

The fired fittings are polarized in a manner known per se, e.g. B. by having two Electrodes (14 and 16 in Fig. 1 and 2) on the opposite end faces of the ceramic body and an electrostatic field is applied to the electrodes. Even if the special ones Conditions for the polarization are changeable as desired, results in a field strength of 60 to 70kV / cm of a constant field at room temperature,

which is maintained for 1 hour to produce perfect results.

Examples of specific ceramic mixtures according to the invention and various useful physical, electrical and electromechanical properties are given in Table I, in which K is the dielectric constant of the material relative to the dielectric constant of the empty space, k " the planar piezoelectric coupling coefficient and% -D the dielectric loss factor measured at 1 kHz and expressed as a percentage.

For comparison, Table I contains values of a control mixture O, which are not the additional Contains reagent according to the invention.

Table I.

10

Basic molar composition Additional Planar number
of the example Reagent coupling Pb Sr (Zr Ti) O in percent by weight (V O Pb _0-94 Sr _0-06 (Zr _0-53 Ti _0i47 ) O ₃ no 0.50 1 Pb _0-94 Sr ₀₁₀₆ (Zr ₀₁₅₃ Ti ₀₁₄₇ ) O ₃ 0.03NiO 0.57 2 Pb _0t94 Sr _0i06 (Zr ₀ a, Ti _0i47 ) O ₃ 0.05 NiO 0.56 3 Pbo.94 S ^r _0- 06 (^ ^Γ 0.531 * 0.47) ^ 3 0.07 NiO 0.57 4th Pbo, 95 S ^r o, O5 (^ ^Γ 0.531 * 0.47) ^ 3 0.10 NiO 0.56 5 Pb _O, 95 S ^r ₀ .05 (^ ^Γ 0:53 0:47 1 **) ^ ₈ 0.70NiO 0.58 6th 1.00NOK 0.58

Table I gives values about the significant properties of mixtures according to the invention which contain nickel oxide as an additive in a range from 0.03 to 1.0 percent by weight. The too The base composition or mixture chosen for the purposes of the example is lead zirconate titanate, the Contains 6 atomic percent strontium as a substitute for the lead. This particular mix is chosen because as explained above, it has a large coercive force which makes polarization relatively difficult. All samples identified in Table I are tested under the same conditions at a field strength of polarized about 40 kV / cm. The significantly higher values of the planar coupling, those with the additional Reagent-containing mixtures are achieved, make the easier polarizability credible compared to the basic mixture. In addition, this benefit is prohibited without an Destruction of other properties of the material achieved.

In FIG. 4, the polarizability (which is indicated as the planar coupling coefficient k " ) of the same lead-zirconate-titanate-based ceramic is shown in the form of a graph

S 1 **

0.47 /

compared with and without the addition of nickel oxide. It should be noted that a higher degree of polarization for all sizes of the polarizing field is achieved; the difference is made at lower field strengths of z. B. 20 kV / cm more noticeable, because at fields above 40 kV / cm the samples approach saturation of polarization. However, it is emphasizes that the use of the additional reagents results in a much higher degree of polarization at lower ones Allows fields and thus allows the polarization of materials that are subject to higher polarization potentials conduct or break through. The advantages of the invention in terms of Achieving higher densities at moderate firing temperatures are compared to the firing properties the same basic mixture according to Table I, namely _, _, _.

shown. When this mixture is formed and ceramic is matured at the most favorable temperature

Table II
_Basic molar mixture: Pb 0j94 Sr _0i06 (Zr _0-53 Ti _0i47 ) O ₃

Oxydic additive
in percent by weight

Firing temperature

( ⁰ C)

Density (g / cm / ⁸ ) dielectric constant,
polarized

Planar coupling coefficient

Electrical loss

No ....
0.3 CoO

0.1 CoO,
0.05 CoO
0.03CoO,

0.1 NiO.

0.05NiO.
0.1 Fe ₂ O ₃

1290

1270 1250 1230 1210

1270 1290 1290

1290 1270 1250

1290 1290

7.35

7.64 7.67 7.65 7.62

7.64 7.59 7.52

7.64 7.64 7.63

7.59 7.61 1300

1250
1450
1450
1340

1250
1120
1300

1020
1250
1310

1190
1010

0.50

0.45
0.46
0.46
0.45

0.45
0.52
0.52

0.54
0.56
0.55

0.56
0.51

0.8

4.0 6.0 6.0 5.0

1.9 1.5 1.0

1.0 0.9 0.8

0.8 0.9

109 738/270

(1280 to 1290 ° C for one hour), the product has a density (determined by immersion in a liquid) of 7.3 to 7.45 g / cm ³ , although the theoretical density is probably about 7.8 . A significant part of the difference between the theoretical and the achieved density is attributable to the open porosity. However, both the theoretical and the achieved density vary considerably according to the composition of the particular mixture.

In the case of mixtures according to the invention which contain additional reagents, which are mentioned above, the degree of sintering achieved during firing is greatly increased, resulting in reduced porosity and consequently gives a higher density. In most cases this is the optimal firing temperature for Mixtures containing the additives are advantageously lower than those without additives. the For example, Table II shows the properties of various blends according to the invention. ao

A look at this table and a comparison with the base mixture shows that the additives result in higher material densities of z. B. 7.5 to 7.7 g / cm ³ at firing temperatures that are the same or lower than those that cause a density of only 7.35 g / cm ³ without additional reagents. These additives do not adversely affect the electrical properties of the material in any way; however, due to the greater density of the body, some properties have been improved.

Claims (8)

PATENT CLAIMS: 1. Use of a mixture of a base material (at least 95 percent by weight), which is such a binary solution of lead titanate (PbTiO ₃ ) and lead zirconate (PbZrO ₃ ) or of lead titanate and lead tannate (PbSnO ₃ ) or such a ternary solution of the three substances mentioned that the composition lies within the square (ABCD) of the triangular mixture diagram (Fig. 3) (in molar ratios) of these three substances, in which the point (A) by 35% PbTiO ₃ , 65% PbSnO ₃ ,
the point (B) by 55% PbTiO ₃ , 45% PbSnO ₃ ,
the point (C) by 60% PbTiO ₃ , 40% PbZrO ₃ and the point (D) is characterized by 10% PbTiO ₃ , 90% PbZrO ₃ , whereby 25 atomic percent of the lead can be replaced by at least one alkaline earth metal, namely by barium, calcium and / or strontium, and from 0.01 to 1.0 percent by weight ( based on oxide) of an additive iron, nickel and / or cobalt after firing until ceramic maturity and after polarization as an electromechanical transducer element. 2. Use of a mixture according to . Spruch 1, whose additive is nickel oxide. - 3. Use of a mixture according to claims 1 and 2, in which up to 5 percent by weight, calculated on the oxide basis, tantalum, niobium and / or a rare earth including yttrium, however, with the exception of cerium, are included. 4. Use of a mixture according to claims 1 to 3, in which at least one element of group VIB of the periodic table is present in an amount that, calculated on the molar basis, an addition of up to 1.5 percent by weight of chromium oxide (Cr ₂ O ₃ ) is equivalent to. 5. Use of a mixture according to claims 1 to 4 with a base material, the composition of which is instead of within the square (ABCD) of the triangular mixture diagram in the square (EFGH) , in which the point (E) by 40% PbTiO ₃ , 60% PbSnO ₃ , the point (F) by 50% PbTiO ₃ , 50% PbSnO ₃ , the point (G) by 50% PbTiO ₃ , 50% PbZrO ₃ and the point (£ 0 by 40% PbTiO ₃ , 60% PbZrO ₃ is marked. 6. Use of a mixture according to Claims 1 to 5, the base material of which is essentially consists of electromechanically sensitive lead zirconate titanate and the amount of additive ranges from 0.01 to 1%. 7. Use of a mixture according to claim 6, wherein the molar ratio of lead zirconate to lead titanate is about 60:40 to 50:50. 8. Use of a mixture according to claim 2, the material of which according to the formula ^Pb 0.94 ^Sr 0.06 ⁱ 0.47) ° 3 is composed and contains 0.03 to 0.07 percent by weight nickel oxide (NiO). 1 sheet of drawings © 109 738/270 10.61