EP0724021B1 - Process for preparing an electro-conductive cermet - Google Patents

Description

The present invention relates to a method for producing an electrically conductive Cermets with a precious metal content of less than 35% by volume by mixing powders a high temperature resistant ceramic and a noble metal, forming a green body the powder mixture and sintering of the green body to form a dense ceramic phase and a contiguous network of metallic phase having cermets. By the term "precious metal" here are gold, silver, and the group The platinum metals platinum, palladium, rhodium, iridium, osmium and ruthenium meant.

Process for the preparation of an electrically conductive cermet with a precious metal content of less than 35 vol .-% by mixing powders of a high temperature resistant ceramic and of a noble metal, forming a green body and sintering the green body into one contiguous network of metallic phase cermets.

Cermets are intimate mixtures of ceramic and metallic components designated. They combine the corrosion resistance and the hardness of ceramic with the electrical conductivity and the strength of metals. They are used for example for electrical feedthroughs for discharge lamps, in spark plugs or for the manufacture used by sensor elements in electrical Massendurchflußmessem.

A generic method for producing such a cermet is known from DE-A1 26 58 647 known. Therein it is proposed, first a dispersion of alumina powder with the addition of chromium nitrate, which is a bonding agent between the ceramic phase and the metal phase to produce. After evaporation of the Dispersion are the individual particles produced with a coating of precious metal, For example, provide platinum by adding a solution of chloroplatinic acid or tetramine-platinum chloride be exposed under the presence of a reducing agent, from which then Platinum on the The green body at approx. 1400 ° C becomes a cermet with good electrical conductivity receive. The electrical conductivity is based on the generated, cohesive, skeletal structure of the metallic phase. The volume fraction of platinum metal is at the known cermet, for example, about 12.5%.

With this method, it is possible to cermets with good electrical conductivity at the same time low platinum content. However, it has been shown that the electrical conductivity such low-cermet cermets after sintering at higher temperatures, above about 1500 ° C, rapidly decreasing. However, for some uses Cermets with high strength or with a gas-tight structure needed. The production of a solid and dense ceramic phase based on high-temperature resistant materials, such as aluminum oxide or zirconia, but requires sintering temperatures of at least 1500 ° C. It has shown that the decrease in electrical conductivity during sintering at high temperatures can be avoided by a higher content of precious metal, for example 40 vol .-%. However, the higher precious metal content is inevitably associated with higher material costs for the cermet connected.

The present invention is therefore based on the object to provide a method which the production of cermets based on high temperature resistant materials with good electrical properties Conductivity and high density with low precious metal content allows.

This object is achieved on the basis of the method described above according to the invention that a noble metal powder is used, which at sintering temperature to form the metallic phase an increase in volume, a decrease in volume or no change in volume that a ceramic powder is used, which at sintering temperature to form the ceramic Phase has a volume decrease, and that in the case that the noble metal powder has a volume decrease, the decrease in volume of the ceramic powder is greater than the decrease in volume of the noble metal powder.
The term "precious metal" is understood to mean gold, silver, ruthenium, rhodium, palladium, osmium, iridium or platinum.
One possible explanation for the decrease in electrical conductivity when sintering low-green-state green bodies at high temperatures would be that the metallic phase contracts on heating to reduce its surface area and reduce surface energy. As a result, for example, fine ramifications of the contiguous metallic structure can be separated. As a result, the conductivity of the cermet decreases. This is avoided by using a noble metal powder with low sintering activity for the formation of the green body. The sintering of the metallic phase causing transport operations that lead to rounding of small radii to a surface reduction of the metallic phase, thereby do not run or only to a small extent. Fine structures of the precious metal realized in the green body are retained even after sintering at high temperatures; fine ramifications of the metallic phase are not interrupted. The reduction in the sintering activity of the noble metal is accomplished by a variety of known means, such as crystal growth inhibiting additives, a narrow particle size distribution of the noble metal powder, a morphology of the individual grains of the powder, which involves low surface energy, or a low specific surface area of the powder as a whole , to reach. Advantageously, the ceramic powder used has a high sintering activity.

Characterized in that the decrease in volume of the metallic phase in the dense sintering of the green body is lower than that of the ceramic phase formed by the ceramic powder takes in the course of dense sintering in the shrinking green body of the metallic phase to Available relative volume from. The ceramic phase shrinks, so to speak, on the Precious metal powder scaffold on. In this case, even present in the green body separated from each other noble metal-containing areas are interconnected. The separation of fine ramifications precious metal-containing areas is prevented. The electrical conductivity of the densely sintered Cermets is therefore higher than that of the green body. This effect is all the more pronounced the more the volume decrease of ceramic and metallic phase differ. To generate the largest possible difference between the respective volume decreases during sintering, a noble metal powder with very small volume decrease and / or a ceramic powder with a very large volume decrease can be selected.

The volume decrease of the powder used for the formation of the green body is determined by means of dilatometer measurements determined. For this purpose, appropriate dilatometer samples are obtained by cold pressing made of precious metal and ceramic powder. These measurements will be under certain In the case of the precious metal powder, an increase in volume on heating has even been observed. The volume increase can be, for example, by relaxation processes of the pre-consolidated Explain samples. In this regard, under the main claim as "small" designated Volume decrease during sintering of the precious metal powder also increases the volume understand.

The noble metal powder has a lower sintering activity than the ceramic powder. The comparison The sintering activities of precious metal powder and ceramic powder is carried out by heating cold pre-pressed samples of the respective powder while observing the grain growth. the one Powder has the higher sintering activity at which grain growth occurs at the lower temperature starts.

Particularly good results are achieved if noble metal powder having a specific surface area, measured by the BET method, of less than 1 m ² / g, preferably less than 0.1 m ² / g, is used. In such a powder, the sintering activity is particularly low due to the low surface energy. Structures and networks in the green body produced with such a powder therefore also remain during sintering at temperatures above 1500 ° C.

It has also proved to be advantageous to use noble metal powder of which 50% by weight. a grain size of less than 20 microns, preferably less than 15 microns, and of the 10 wt .-% have a particle size of at least 2 microns, preferably at least 4 microns. Such a powder has a relatively narrow particle size distribution and one for a slow one Sintering cheap average grain size. Very small grains are avoided as possible due to their small radii they have a high surface energy and thus a high sintering activity exhibit. Very large starting grains besides smaller grains can be a fortified Grain growth, a so-called giant grain growth, experienced in which the areas around the Impoverish "giant grains" of precious metal. This precious metal depletion can lead to separations in lead the filigree, metallic network structure. A narrow particle size distribution is reduced the sintering speed in addition.

A method is preferred in which a ceramic powder is used, the specific Surface, measured by the BET method, is larger by at least a factor of 20 as the specific surface of the noble metal powder. The specific surface is a measure of the sintering activity. For a ceramic powder with a relatively large specific surface area in comparison to the noble metal powder, a higher sintering activity is expected. This is an early one Volume decrease of the ceramic phase ensured.

In this regard, it has also proven to be advantageous to use a ceramic powder with a medium Grain size, which is at least ten times smaller than that of the noble metal powder use, wherein at least 90 wt .-% of the ceramic powder has a grain size of not more than 5 have μm.

The use of noble metal powder has proven to be particularly favorable, in which the volume decrease the metallic phase in the densification of the green body by at least 5%, preferably 10% less than that of the ceramic phase. As it is only on the difference the specific volume shrinkage arrives, the selection of suitable starting materials be turned off on the ceramic powder instead of the precious metal powder. As at the beginning already explained, under some circumstances, the observed with the noble metal powder Volume shrinkage will be zero or even negative.

Advantageously, a ceramic powder is used, in which the volume decrease of the ceramic Phase at a lower temperature than the volume decrease in the metallic Phase. This ensures that the metallic phase at any time during dense sintering, a relative volume within the green body is available is greater than its relative initial volume. The tearing of fine ramifications the metallic phase is thus prevented.

Has proven to be a procedure in which as precious metal powder platinum powder is used, in which the high temperature resistant ceramic contains alumina, and in the at temperatures between 1500 ° C and 1750 ° C, preferably just below the Melting point of platinum is sintered. This creates a particularly dense cermet. It has been found that with such a process, even with platinum levels down to 25 vol .-%, densely sintered cermets with a very high electrical conductivity produced are. The preferred sintering temperatures are around 1700 ° C.

Figur 1

ein Binärbild eines Schliffes von einem handelsüblichen Cermet,

Figur 2

ein Binärbild eines Schliffes von einem nach dem erfindungsgemäßen Verfahren hergestellten Cermets,

Figur 3

eine statistische Auswertung des in Figur 2 dargestellten Schliffbildes,

Figur 4

eine weitere statistische Auswertung des in Figur 2 dargestellten Schliffbildes,

Figur 5

ein weitere statistische Auswertung des in Figur 2 dargestellten Schliffbildes und

Figur 6

das Ergebnis einer dilatometrischen Messung bei einem für die Herstellung des erfindungsgemäßen Cermets eingesetzten Keramik-Pulver und bei einem Edelmetall-Pulver.

An embodiment of the invention is illustrated in the drawing and will be explained in more detail below. In the drawing show in detail

FIG. 1

a binary image of a cut from a commercial cermet,

FIG. 2

a binary image of a cut of a cermet produced by the method according to the invention,

FIG. 3

a statistical evaluation of the microsection shown in Figure 2,

FIG. 4

a further statistical evaluation of the microsection shown in Figure 2,

FIG. 5

a further statistical evaluation of the microsection shown in Figure 2 and

FIG. 6

the result of a dilatometric measurement in a ceramic powder used for the production of the cermet according to the invention and in a noble metal powder.

In the binary images of both cuts according to Figures 1 and 2, the ceramic phase black and the metallic phase reproduced in white. When the ceramic phase is each is alumina, in the metallic phase to platinum.

The cermet according to the prior art according to FIG. 1 contains about 40% by volume of platinum. The ceramic phase consists essentially of Al ₂ O ₃ and is densely sintered. The sintering temperature of this cermet should therefore have been above 1650 ° C.

A striking feature of the microsection is the broad size distribution of the cut surfaces of the metallic one Phase. In particular, some very large areas can be seen. These big contiguous Areas of metallic phase have very many pores. Furthermore, it can be seen that the individual areas of metallic phase with a plurality of sharp edges, or very small radii are provided. Apparently was for the production of this cermet used a powder with a very high sintering activity. The high sintering activity could for example, to the concentration of metallic phase in the mentioned very large Areas. These areas contribute to the electrical conductivity of the cermet not essential. On the contrary, they degrade the given platinum content Conductivity, because in them, the conductive material is concentrated and accordingly to others Job is missing. Furthermore, the uneven distribution of the apparent from FIG metallic phase because of differences in the thermal expansion coefficient of Ceramic and metal also stress within the cermet and therefore leads to a Strength reduction.

The cermet, the micrograph of which is shown in binary form in Figure 2 , has a platinum content of 30% by volume; the remainder consists essentially of alumina. The green body mixed and molded from the starting powders was densely sintered at 1700 ° C.

Compared to the grinding of FIG. 1, the binary image of FIG. 2 shows the more uniform distribution of the metallic phase in the ceramic phase. For a high electrical conductivity, it has proven to be advantageous if the cut surfaces of the metallic phase, as shown in the micrograph of Figure 2, an area of at most 1000 .mu.m ² , preferably less than 800 .mu.m ² and if the curve of Area distribution drops very steeply from its maximum to larger values. Such a narrow size distribution of the cut surfaces of metallic areas in the sectional image is an indication of a homogeneous distribution and a finely branched structure of the metallic areas in the cermet.

Furthermore, it can be seen from a comparison with Figure 1, that the areas with metallic phase in the cermet according to FIG. 2 by an overall somewhat rounded shape and in particular characterized by rounded edges. This is a sign of a low Sintering activity of the starting powder. There are only a few pores to be recognized.

The above statements are by the illustrated in Figures 3 to 5 statistical Bildananlysen underpins:

FIG. 3 shows the result of a statistical image analysis on the distribution of the metallic phase of the micrograph shown in FIG. 2 on the basis of a histogram. On the X-axis of the histogram in the unit μm the length of the outer boundary line of the areas with metallic phase, divided into length classes, is plotted; the Y axis denotes the absolute frequency of the respective length classes. Thereafter, the maximum of the frequency is at a circumferential length of about 16 microns, the frequency distribution decreases slightly slower in the direction of the smaller lengths and slightly slower in the direction of the longer lengths. Overall, however, the frequency distribution is relatively narrow. The mean value of the frequency curve is 32 μm. From the information below the diagram shown, the frequencies in the respective length classes are listed in detail.

FIG. 4 also shows, in the form of a histogram, the area fraction of metallic phase in a total of 9 statistically selected image sections. The histogram illustrates impressively that in all selected image sections the area covered by the metallic phase is almost constant at 29%. This too is an indication of the uniform distribution of the metallic phase.

In the image analysis according to FIG. 5 , a further frequency distribution, likewise in the form of a histogram, is shown. Here, the X-axis denotes the distance of adjacent areas with metallic phase in μm and the Y-axis the absolute frequency of the respective distance classes. According to this, the maximum frequency for a distance class is in the range from 289 to 394 μm. The mean distance is given as 260 μm. The frequency distribution drops steeply at greater distances and flatter at shorter intervals. Overall, however, the frequency distribution is very narrow.

The statistical image analyzes shown in FIGS. 3 to 5 prove the uniform distribution of the metallic phase in the cermet according to the invention.

FIG. 6 shows the result of a dilatometric measurement of Al ₂ O ₃ powder and platinum powder used to produce a cermet according to the invention. To carry out the dilatometer measurement, compacts were produced from the powders by cold pressing. The length of the Al ₂ O ₃ powder compact was 39.31 mm, that of the platinum powder compact 23.48 mm.

The time in minutes is plotted on the x-axis of the diagram shown in FIG. 6, the temperature in ° C. on the left y-axis, and the length changes measured on the compacts on the right-hand y-axis. In the diagram, a first rapid heating phase with phase 1, a subsequent slower heating phase with phase 2 and a constant high temperature region at about 1600 ° C. are designated phase 3. The corresponding temperature profile is indicated by the reference numeral 4 in the diagram. Furthermore, measured in the diagram, the measured on the Al ₂ O ₃ powder compact expansion curve 5 and the measured on the platinum compact expansion curve 6.

From the course of the expansion curve it can be seen that the beginning of the volume decrease in the Al ₂ O ₃ powder compact is at a temperature of about 1400 ° C. As the temperature increases, the length of the compact decreases rapidly, indicating rapid sintering. Overall, the irreversible length shrinkage of Al ₂ O ₃ -Presling is 13.8%.

The extension trace 5 of the platinum powder compact shows no decrease in length increasing temperature. On the contrary, it became around 1580 despite high temperature sintering ° C detected an irreversible increase in length of about 6%. It follows that also at the maximum temperature of the dilatometer measurement in the platinum powder used no sintering has, as in this case a length decrease of the sample should have been observed.

The results of the dilatometer measurements according to FIG. 6 make it clear that the platinum powder used for the production of the cermet is distinguished from the Al ₂ O ₃ powder by a lower sintering activity, as a result of which no decrease in volume is observed at sintering temperatures around 1600 ° C. , In contrast, the Al ₂ O ₃ powder used for the production of the cermet has a very significant volume shrinkage at this temperature. Consequently, even with a homogeneous mixture of the two powders, the ceramic Al ₂ O ₃ phase sinters more rapidly, virtually shrinking onto the three-dimensional platinum framework in the green body, solidifying it and causing the conductivity of the cermet.

In the following, suitable starting powders are used for the production of a cermet according to the invention explained in more detail with reference to a further embodiment.

To form the green body with 25 vol% platinum, balance Al ₂ O ₃ , a platinum starting powder was used, which had a BET surface area of 0.06 m ² / g. Its average grain size was 10 microns. About 80% by weight of the platinum powder was in the particle size range between 4 μm and 20 μm. Overall, the platinum powder is characterized by a very low sintering activity. The structure obtained with it once in the green body therefore essentially remains even when sintered at 1700 ° C.

The Al ₂ O ₃ starting powder used had an average particle size of about 1 μm. 90% by weight of the starting Al ₂ O ₃ powder had a particle size of less than 3 μm. Its BET surface area is 4 m ² / g. The Al ₂ O ₃ starting powder is characterized by a significantly higher sintering activity compared to platinum powder. It has also been found that during dense sintering, the ceramic phase formed from the Al ₂ O ₃ starting powder experiences a substantially greater volume decrease than the metallic phase formed from the platinum powder. In this case, a noticeable decrease in volume occurs at the ceramic phase at a temperature of about 1400 ° C, while no change in volume is observed in the metallic phase. These differences in volume changes in the two starting coils further contribute to stabilizing the structure formed by the metallic phase in the green body and even solidifying it by shrinking the ceramic phase onto the metallic phase. This results in a network-like, substantially finely branched structure of contiguous platinum-containing areas, which leads to a high electrical conductivity in the densely sintered cermet.

When produced by the process according to the invention densely sintered, tablet-shaped Cermets with a platinum content of 25 to 30 vol .-% and with a diameter of about 10 mm, over its thickness of 6 mm, an electrical resistance of less than 10 ohms measured.

Claims (8)

1. Process for the production of an electrically conductive cermet with a noble metal content of less than 35% by vol. By mixing powders of a ceramic material resistant to high temperatures and a noble metal, forming a green body from the powder mixture and centring of the green body with the formation of a dense ceramic phase and a cohesive network of cermets exhibiting a metallic phase, a noble metal powder used which, on centring, has a lower centring activity that the ceramic powder and, with the formation of the metallic phase, exhibits an increase in volume, a decrease in volume or no change in volume, a ceramic powder being used which exhibits a decrease in volume of centring with the formation of the ceramic phase, and that in the case that the noble metal powder exhibits a decrease in volume, the decrease in volume of the ceramic powder is greater than the decrease in volume of the noble metal powder.
2. Process according to claim 1 characterised in that a noble metal powder with a specific surface area, measured according to the BET method, of les than 1 m²/g, preferably of less than 0.1 m²/g is used.
3. Process according to claim 1 or 2 characterised in that noble metal powder with an average grain size of at least 10 µm, preferably at least 20 µm is used, maximum 10% by weight of the noble metal powder having a grain size of less than 2 µm.
4. Process according to one of the preceding claims characterised in that a ceramic powder is used whose specific surface area measured according to the BET process exceeds the specific surface area of the noble metal powder by at least a factor of 20.
5. Process according to one of the preceding claims characterised in that a ceramic powder is used with an average grain size which is at least 10 times smaller than that of the noble metal powder, at least 90% by weight of the ceramic powder having a grain size of maximum 5 µm.
6. Process according to one of preceding claims characterised in that a noble metal powder is used in the case of which the decrease in volume on centring is by at least 5%, preferably by at least 10% less than the corresponding decrease in volume of the ceramic powder on formation of the ceramic phase.
7. Process according to one of preceding claims characterised in that a ceramic powder is used in the case of which the decrease in volume begins at a lower temperature than the decrease in volume of the noble metal powder.
8. Process according to one of preceding claims characterised in that platinum powder is used as noble metal powder, that the ceramic material resistant to high temperature contains aluminium oxide and that centring is carried out at temperatures between 1500°C and 1750°C, preferably just below the melting point of platinum.

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