CH622484A5 - Process for producing a sintered ceramic product - Google Patents

Description

The present invention relates to a method for producing a sintered ceramic product.

The Japanese Journal of Applied Physics for 1972 describes sintering silicon nitride, aluminum nitride and alumina to produce a solid solution. At least a small amount of aluminum oxynitride appears to be present in accordance with the temperatures and time periods specified therein. However, it is not disclosed in this reference that two or more glass-forming oxides other than magnesium oxide and alumina only are included to produce a low melting temperature glass to aid in the densification of the ceramic.

A method according to the present invention is characterized in that aluminum nitride or aluminum oxynitride and silica or silicon oxynitride are reacted with one another at a temperature between 1200 and 2000 ° C. to form a silicon-aluminum oxynitride ceramic material, the mixture being at said temperature contains no more than 60% by weight of aluminum nitride and first and second metal oxides, including boron oxide and excluding silica, the metal oxides being present in such a way that they do not introduce a combination of only magnesium oxide and alumina into the mixture, and

5

10th

15

20th

25th

30th

35

40

45

50

55

60

65

3rd

622 484

that they combine with some of the silica during the reaction to form a silicate glass which has a liquidus temperature lower than the liquidus temperature of a silicate glass formed from silica with one of the metal oxides alone, at least one of the metal oxides can be replaced by a silicate, which delivers the oxide at the temperature mentioned.

Only magnesium oxide and alumina are excluded because alumina is a substance that allows aluminum and oxygen to penetrate the silicon nitride crystal lattice in non-stoichiometric proportions so as to produce silicon aluminum oxynitrides, and because magnesium oxide does not enter the silicon nitride -Crystal lattice enters. This means that if only magnesium oxide and alumina are used as metal oxides, it is not possible to control the melting point of the glass, which is a basis of the present invention.

The ceramic material mentioned preferably contains 95% by weight of a compound according to the formula:

SÌ6_zAlzNg_zOz where z is greater than zero and less than or equal to 5.

An advantageous method is characterized by a sintering process at a temperature between 1200 and 2000 ° C., by a mixture which contains between 15 and 45% by weight of silica, between 0.05 and 50% by weight of alumina and between 40 and 60% by weight Aluminum nitride contains, in such relative proportions, that at the temperature mentioned a ceramic material is formed which contains at least 95% by weight of a compound according to the formula:

SÌ6_zAlzNg_zOz contains, where z is greater than zero and less than or equal to 5, and the mixture also contains at least 2 metal oxides, other than silica, which are arranged such that they are present during heating with part of the silica which is present in the mixture is to react to form a silicate glass which has a liquidus temperature below that of the silicate which would be formed from silica with one of the metal oxides alone, and which glass contributes to the densification of the ceramic material.

It is advantageous if the mixture also contains nitrogen silicide at the temperature mentioned and that the relative proportions of the nitrogen silicide, clay, aluminum nitride and silica in the mixture are such that a reactive composition is formed at the temperature mentioned which is at least 95 % By weight of the mixture mentioned and in which the atomic ratio of

Silicon: aluminum: nitrogen: oxygen is 6-z: z: 8-z: z, where z is greater than zero and less than or equal to 4, and that the components of the reactive compound mentioned react with one another to form the ceramic material mentioned .

Preferably, at least part of the silica is present in the mixture at the temperature mentioned as an impurity of the nitrogen silicide.

The process can preferably be characterized in that the mixture contains no nitrogen silicide and in that the relative proportions of the alumina, aluminum nitride and silica in the mixture are such that a reactive composition is formed at the temperature mentioned which comprises at least 95% by weight of the mixture forms and in which the atomic ratio of

Silicon: aluminum: nitrogen: oxygen is 6-z: z: 8-z: z, where z is greater than 4 and less than or equal to 5, and that the components of the reactive composition react with one another to form the ceramic material mentioned.

At least part of the alumina is preferably present in the mixture at the temperature mentioned as an impurity of the aluminum nitride.

The liquidus temperature of the silicate which is formed from silica together with the metal oxides is preferably at least 100 ° below that of the silicate which would be formed from silica with each metal oxide alone.

The heating process is preferably accompanied by pressure.

It is advantageous if the metal oxides are printing aids for the ceramic material mentioned and their total amount in the mixture is less than the amount which would be required if each metal oxide were present alone.

The heating process can also take place without pressure.

At least one of the components which are present in the mixture at the temperature mentioned is preferably added to the starting materials which are used as a compound for the preparation of the mixture and which provide the required component or components at the temperature mentioned.

At least one of the metal oxides can be introduced into the mixture mentioned as a metal compound which can decompose into the required oxide during the heating process.

It is advantageous if the metal oxides are selected from a group consisting of magnesium oxide, alumina, manganese oxide, lithium oxide, titanium dioxide, boron oxide and iron oxide.

For reasons of comparison, a control experiment was first carried out in which nitrogen silicide, aluminum nitride and silica in powder form were mixed in the required amounts in order to produce a ceramic material which essentially finally consists of a compound according to the general formula:

Si6_zAlzN8._zOz where z was 1.

The nitrogen silicide powder used consisted of 89% a-phase material and had an average particle size of 3 microns. The aluminum nitride powder used was that of the Koch-Light company, type "8006H", which had an average particle size of 11.5 microns, but which was reduced to an average particle size of 7 microns before use in a colloidal mill. In addition, that of Hopkin and Williams Limited was used as the pure powdered silica. It was known that the nitrogen silicide powder inevitably contained silica as a coating over the particles of the nitrogen silicide, and further that the aluminum nitride inevitably contained alumina contamination. It is understandable that these two contaminants affect the subsequent reaction to produce a silicon aluminum oxynitride ceramic material because they bring silicon, aluminum and oxygen into the reaction. Therefore, before the starting materials were mixed, the degree of contamination in the nitrogen silicide and in the aluminum nitride was determined by means of the rapid neutron activation analysis, and the impurities were then calculated in the required composition for the mixture. When using the special starting materials described above, it was found that the silica content of the nitrogen silicide powder was 4% by weight and the alumina content of the aluminum nitride was 6% by weight. From this it was calculated that in order to

10th

15

20th

25th

30th

35

40

45

50

55

60

65

622 484

4th

To obtain ceramic material, the necessary composition for the starting mixture is 78.48% by weight nitrogen-silicide powder, 14.76% by weight aluminum nitride powder and 6.76% by weight silica powder, since the impurities in the nitrogen silicide and aluminum nitride including this mixture of silicon, aluminum, nitrogen and oxygen in the required proportions:

6-z: z: 8-z: z, where z was 1.

A first metal oxide in the form of a magnesium oxide powder, which is a known hot pressing aid for the nitrogen silicide ceramic material, was added to the mixture mentioned above. The magnesium oxide powder was supplied by Hopkin and Williams Limited under the name "light" and the amount of magnesium oxide that was added was such that it was 1% by weight of the total mixture. The total composition of the starting mixture was thus 74.6% by weight of nitrogen silicide, 13.72% by weight of aluminum nitride, 9.8% by weight of silica, 0.88% by weight of alumina and 1% by weight of magnesium oxide.

After the magnesium oxide powder was added, the starting mixture was placed in a colloid mill and mixed in an isopropyl alcohol until the average particle size of the mixture was 3 microns. The mixture was then dried and then sieved to remove powder agglomerates. A contamination determination was then carried out, which showed that the milling process, as well as the drying and the screening, had no influence on the degree of contamination of the starting materials. The mixture was then placed in a graphite mold and this was sealed. The graphite mold was then assembled onto the powder batch, all graphite surfaces coming into contact with the powder, which was first sprayed with boron nitride to a depth of the order of 0.25 mm. The whole was then placed in a press where the temperature was simultaneously raised under pressure over a period of 30 minutes to 1800 ° C and 23.1 x 108 Pa. The mixture was then held at that temperature and pressure for one hour, and under these conditions the mixture consisted entirely of magnesium oxide together with a reactive composition in which the atomic ratio of silicon: aluminum: nitrogen: oxygen corresponded to the proportions given above. which of course were the required proportions in the ceramic material to be produced. Thus, the components of the reactive composition reacted together to form the required ceramic material, the hot pressing of which was favored by the magnesium oxide. On cooling, the hot pressed product was removed from the mold and subjected to an X-ray analysis which showed that the ceramic phase of the product consisted entirely of a compound which had a z-value of 1 in the formula given above. The product had a density of 3.16 gm / c.c. and it should be noted that during the hot pressing the mixture has reached 90% of its final density when the temperature reaches approximately 1650 ° C. In addition, the product had a breaking stress at room temperature corresponding to 4.13 • 109 Pa and a Weibull distribution of 8.0.

The breaking stress is defined as a value of the tension in the outer fibers of a ceramic product under tension. This test is carried out using an elongated test rod with a rectangular cross-sectional shape. The flat side of the rod is placed at two points at a distance of 1. The shorter side of the

Rectangle of the cross-sectional shape of the rod as d and the longer side (on which the rod rests) as b. W is the force applied in the middle of the resting bar. The rod is placed on wedges, ie the support points are linear, and the force W is also applied by means of a wedge, so it also acts on a linear area.

The Weibull distribution in this case is:

1.2 • Average breaking stress, standardized deflection

In a first example according to the present invention, the starting mixture of the control experiment was used again, with the difference that in this example the magnesium oxide was reduced to 1 to 0.5% by weight and that a second metal oxide in the form of a 0.5 % By weight of manganese oxide (Mn304) have been used in the mixture. The manganese oxide used was supplied by Hopkin and Williams Limited and in this example the process of the control experiment was followed. When removed from the mold, the product had an average breaking stress at room temperature of 5.65-109 Pa and a Weibull distribution of 10 and a density of 3.23 gm / c.c. on. In addition, it was noted that during the hot pressing process, the sample reached 90% of its final density when the temperature reached approximately 1450 ° C, which was a lower temperature than that of the control test.

In the process of the first example, manganese oxide and magnesium oxide reacted with silica which was present in the starting mixture to produce a magnesium-manganese silicate glass whose liquidus temperature was below that of the silicate which would have been formed if magnesium oxide had been used alone . Thus, during the hot pressing, magnesium manganese silicate was provided in a less viscous liquid glass form than that which would have been formed from the magnesium silicate of the control example at the same temperature. The magnesium-manganese-silicate glass was intended for easier compaction of the material and resulted in an improved consistency, strength and density in the finished product.

In a second example according to the invention, the procedure of the first example was repeated, but in this case magnesium oxide and manganese oxide were added to the starting mixture in an amount of 1% by weight of the total mixture, while nitrogen silicide was reduced to 1% by weight. Taking into account the impurities of nitrogen silicide and aluminum nitride, the starting mixture consisted of 73.6% by weight nitrogen silicide, 13.72% by weight aluminum nitride, 9.8% by weight silica and 0.88% by weight. Alumina, 1% by weight magnesium oxide and 1% by weight manganese oxide. The finished product had an average tensile strength at room temperature of 6.55-109 Pa, a Weibull distribution of 8.4 and a density of 3.24. Again, a density of 90% was reached at a temperature of approximately 1450 ° C during the hot pressing process.

In a third example, the process of the first example was repeated, but in this case the manganese oxide content was 1% by weight of the total mixture without increasing the magnesium oxide content. In addition, the nitrogen silicide content was reduced to 0.5% by weight in order to increase the magnesium oxide, the total composition of the starting mixture now being 74.1% by weight nitrogen nitride, 13.72% by weight aluminum nitride, 9.8% by weight of silica, 0.88% by weight of alumina, 0.5% by weight of magnesium oxide and 1% by weight of manganese oxide. The finished product had an average tensile strength at room temperature of 6.89-10® Pa,

5

10th

IS

20th

25th

30th

35

40

45

50

55

60

65

5

622 484

a Weibull distribution of 9.0 and a density of 3.2 gm / c.c. on. Again, a density of 90% at a temperature of 1450 ° C was reached during the hot pressing process.

In a fourth example, the magnesium oxide of the third example was reduced by 0.25% by weight and the nitrogen silicide was increased by the same amount. The process of the control experiment was repeated and a product resulted which had an average tensile strength at room temperature of 7.23 · 108 Pa, a Weibull distribution of 12 and a density of 3.21 gm / c.c. exhibited. Again it was found that during the hot pressing process the material 90%

reached its final density at around 1450 ° C.

It is believed that the improved results of the method of the fourth example were obtained because the relative proportions of the magnesium oxide and the manganese oxide were such that a magnesium-manganese silicate was formed with a very low liquidus temperature, possibly on the order of 1200 ° C . It is also believed that small amounts of magnesium and / or manganese have penetrated the silicon aluminum oxynitride lattice.

In a fifth example, the process of the first example was repeated, but in this case the magnesium oxide was reduced to 0.05% by weight and the manganese oxide was reduced to 0.2% by weight, the nitrogen silicide being in proportion to the decrease in Amounts of oxide additives have been increased. Thus the total composition of the starting mixture of the fifth example was 75.6% by weight of nitrogen silicide, 13.72% by weight of aluminum nitride, 9.8% by weight of silica, 0.88% by weight of alumina, 0.05% by weight. % Magnesium oxide and 0.2% by weight manganese oxide. The finished product from this mixture had an average tensile strength at room temperature of 3.79 • 108 Pa, a Weibull distribution of 7.0 and a density of 2.78. In addition, the material reached 90% of its final density at a temperature of approximately 1500 ° C during the hot pressing process. When the product was subjected to a creep test at 1225 ° C and a load of 1.54-107 Pa was applied, a 0.103% creep observed in 100 hours.

In a sixth example, the mixture was hot pressed and again contained the same amount of the first and second metal oxides used in the first example. Since different batches of silicon nitride and aluminum nitride powder were used in this example, the relative proportions of the starting mixture have been changed. The mixture now consisted of 83% by weight of nitrogen silicide, 10% by weight of aluminum nitride, 6% by weight of silica and 0.5% by weight of magnesium io and manganese oxide. The mixing and hot pressing of the control experiment were repeated, and after removal from the mold the product had an average tensile strength at room temperature of 7.03 x 108 Pa, a Weibull distribution of 10 and a density of 3.23 gm / c.c. on. 15 During the hot pressing process it was found that the sample had again reached 90% of its final density at a temperature of approximately 1450 ° C.

In the case of the starting mixture of the sixth example, an analysis showed that the silica content of the nitrogen-silicon dioxide powder was 2.6% by weight and that the alumina content of the aluminum nitride powder was 4.25% by weight. From these numbers it can easily be calculated that at the hot pressing temperature of this starting mixture showed a reactive composition, which of silicon, aluminum, 25 nitrogen and oxygen was in the atomic ratio of 6-z: z: 8-z: z, where z 0 , 8 corresponded, but which was only about 90% by weight of the total mixture. As expected, the sintered product of the sixth example contained 5% by weight of a glassy phase in addition to the silicon aluminum oxynitride 30 ceramic material, which corresponded to the formula Ss ^ Alo.sNy ^ Oo.s, with little weight loss being noted.

In a modification of the sixth example, the same process was repeated with three different starting mixtures containing different amounts of the magnesium and 35 manganese oxide powders, the nitrogen silicide content of the mixtures being adjusted where necessary to accommodate the changes in the Balance amounts of metal oxides. The results of this modification are shown in the table below.

% By weight of the composition of the starting mixture

Si3N4 A1N Si02 MgO Mn304

Properties of the hot pressed product

Medium Weibull density

Break stress distribution

82

82.5

82.75

10 10 10

6 6 6

1

0.5 0.25

7.58 • 108 Pa 7.58 • 108 Pa 7.85 • 108 Pa

8.4 7.2 20.8

3.24

3.20

3.21

From this table it can be seen that the best results were obtained in the fourth example when the mixture contained 0.25% by weight of magnesium oxide and 1% by weight of manganese oxide.

In a seventh example, the starting mixture of the control example was used, but the magnesium oxide was reduced from 1% by weight to 0.5% by weight and 0.5% by weight of titanium dioxide was added to the mixture for this purpose. The process of the control experiment was then repeated, and when the product was removed from the mold, it had an average breaking strength at room temperature of 6.2 · 108 Pa, a Weibull distribution of 12.6 and a density of 3.19 gm / c.c. on. In addition, it was found during hot pressing that the sample had reached 90% of its final density at a temperature of approximately 1350 ° C, which of course represents a lower temperature than that in the control example. When the product was subjected to a creep test at 1225 ° C. with a load of 7.72 • 107 Pa, it was found that the product had a creep value of 0.10% in 20 hours.

50 In an eighth example, the procedure of the previous example was repeated, but in this case the amounts of magnesium oxide and titanium dioxide in the starting material were increased to 1% by weight, while the nitrogen silicide was reduced by 1% by weight. The starting mixture thus consisted of 73.6% by weight of nitrogen silicide, 13.72% by weight of aluminum nitride, 9.8% by weight of silica, 0.88% by weight of alumina, 1% by weight of magnesium oxide and 1% by weight Titanium dioxide. The finished product had an average tensile strength at room temperature of 5.2 • 108 Pa, a Weibull distribution of 60 8 and a density of 3.18. In addition, it was found that during hot pressing, the material reached 90% of its final density at a temperature of approximately 1550 ° C.

Although only two metal oxides have been added in each of the examples to produce the required silicon aluminum oxynitride, more than two metal oxides can also be used. Thus, in a ninth example, the procedure of the previous examples is repeated

622 484

6

fetches with a starting mixture consisting of 10% by weight aluminum nitride, 6% by weight silica, 83.7% by weight nitrogen silicide, 0.1% by weight magnesium oxide, 0.1% by weight lithium oxide and 0.1% by weight Boron oxide (B203). In this mixture, the magnesium oxide and lithium oxide form first and second metal oxides as in the seventh example and were used in the same weight ratios because they react with part of the silica present to produce a glass with a low melting point. The third metal oxide, that is boron oxide, should reduce the melting point of the glass. The boron oxide used was prepared by calcining the orthoboric acid at 800 ° C, and the resulting oxide was crushed and mixed with the other starting materials in a colloid mill. With this mixture, it was found that the ceramic phase of the hot-pressed product essentially consisted entirely of a compound with a z-value of the order of 0.8 in the above-mentioned formula. The product had an average tensile strength at room temperature of 6.2 • 108 Pa and a Weibull distribution of 10. When the sintered product was subjected to a creep test at 1227 ° C with a load of 1.54-107 Pa, 0.05% creep was found in 100 hours.

In a modification of the ninth example, the process was repeated, but with 0.1% by weight of a fourth metal oxide in the form of alumina, which was added to the starting mixture, the nitrogen silicide of the mixture being reduced by 0.1% by weight is. It was found

that the addition of alumina further favors the densification of the ceramic material during the sintering process.

In a tenth example, three metal oxides were added to the starting mixture, which in this case consisted of 11% by weight aluminum nitride, 6% by weight silica, 80.75% by weight nitrogen silicide, 0.25% by weight magnesium oxide, 1% by weight. % Manganese oxide (Mn304) and 1% by weight iron oxide. The hot pressing process of the control experiment was repeated and, as in the previous example, the ceramic phase of the product consisted essentially entirely of a compound with a z-value of approximately 0.8. The product had an average tensile strength at room temperature of 7.23 • 108 Pa and a Weibull distribution of 20.

It should be noted that each of the examples mentioned above could have been carried out without applying pressure during the sintering process. Thus, in an eleventh example, a sample of the starting mixture of the preceding example is used which has been placed in a rubber bag and has been isostatically pressed at room temperature and a pressure of 3.39-1010 Pa to a density of 1.5 gm / c.c. to create. This sample was then coated with a mixture consisting of equal parts by weight of boron nitride and silica and was placed in a bed of boron nitride powder in the graphite pot. The whole was then heated at 1700 ° C for one hour, after which the temperature was raised to 1800 ° C and held at that temperature for another hour. The sintered product had an average tensile strength at room temperature of 3.1 • 108 Pa, a Weibull distribution of 12 and a density of 2.55 gm / c.c.

In each of the examples mentioned above, the ceramic phase of the sintered product consisted entirely or predominantly of a compound which corresponded to the formula given and had a z value equal to or less than 1. However, the method described can also be used to produce ceramic materials with z values up to 5 in the above-mentioned formula. In a twelfth example, a ceramic material with a z-value equal to 4 is produced in the above formula by sintering a starting mixture which consists of 38% by weight aluminum nitride, 22.5% by weight silica, 22.5% by weight alumina, 16% by weight of nitrogen silicide and 1% by weight of lithium oxide consisted of. In this mixture, part of the alumina formed one of the glass-forming metal oxides, while the rest reacted at the elevated temperature of the sintering process to produce the required ceramic material. The lithium oxide was added to the starting material mixture in the form of a lithium silicate as in the seventh example, so that part of the silica in the starting mixture was supplied by the lithium silicate. When the starting mixture was hot pressed according to the procedure in the control experiment, the resulting sintered product had an average tensile strength at room temperature of 3.1 x 108 Pa, while when the mixture was sintered without applying pressure according to the procedure described in the twelfth example, the sintered product Product had an average tensile strength at room temperature of 2.41 • 108 Pa and a Weibull distribution of 8.

Although in the above examples the heated mixtures contained nitrogen silicide, the process described can also be carried out with mixtures which contain only silica, alumina, aluminum nitride and two or more metal oxides. However, where nitrogen silicide is not present in the starting mixture, it has been found that the reactive composition always has a z-value above 4, although the atomic ratio of silicon: aluminum: silicon: oxygen is 6-z: z: 8-z : z is. A ceramic material thus obtained has a z-value greater than 4.

In each of the starting mixtures of the examples described, the total amount of metal oxides was 2% by weight of the mixture and below. However, larger amounts of metal oxides can also be used, in particular where the sintering is carried out without pressure in order to promote compaction. The metal oxide content is usually such that weight losses during sintering are taken into account and the amount of silicate glass in the sintered product does not exceed 5% by weight of the product. However, this is not an absolute limit, as higher glass contents are tolerable, especially if the product is used at low temperatures and in circumstances where creep resistance is not important (e.g. corrosion-resistant material).

When carrying out the described processes, it is of course possible for one or more of the starting materials to contain metal oxides as impurities, which are reactive with silica to produce a silicate glass with a low melting point. It is therefore necessary that such impurities be included in the calculations of the amounts of metal oxides added to the starting materials.

It is understandable that when the process according to the invention is carried out, the sintering of the starting materials should be carried out at or above 1200 ° C., since there is little or no reaction at this temperature to produce the desired ceramic material. However, the sintering temperature should not exceed 2000 ° C, since above this temperature at least some of the samples show a tendency to dissolve. The optimal temperature is therefore between 1500 ° C and 1800 ° C, because here a reasonable reaction speed takes place without great weight loss. However, it is understandable that even if the sintering temperature is within the optimal range, certain weight losses are unavoidable, e.g. B. the lithium oxide in the ninth and twelfth examples, and these should be taken into account in the calculations of the starting materials.

s

10th

15

20th

25th

30th

35

40

45

50

55

60

65

S

Claims (19)

622 484 2nd PATENT CLAIMS 1. A process for producing a sintered, ceramic product, characterized in that aluminum nitride or aluminum oxynitride and silica or silicon oxynitride are reacted with one another at a temperature between 1200 and 2000 ° C. to form a silicon-aluminum-oxynitride ceramic material the mixture containing at the temperature mentioned not more than 60% by weight of aluminum nitride and first and second metal oxides, including boron oxide and excluding silica, the metal oxides being present in such a way that they do not introduce any combination of only magnesium oxide and alumina into the mixture, and that they combine with a portion of the silica during the reaction to form a silicate glass having a liquidus temperature lower than the liquidus temperature of a silicate glass formed from silica with one of the metal oxides alone, wherein at least one of the metal oxides can be replaced by a silicate, that delivers the oxide at the temperature mentioned. 2. The method according to claim 1, characterized in that aluminum nitride and silica are reacted at a temperature between 1200 and 2000 ° C to form a silicon-aluminum oxynitride ceramic material, the mixture at said temperature not more than 60 % By weight of aluminum nitride and first and second metal oxides, including boron oxide and excluding silica, are present, the metal oxides being present in such a way that they do not introduce a combination of only magnesium oxide and alumina into the mixture, and that they react during the reaction with part of the Join silica to form a silicate glass that has a liquidus temperature that is lower than the liquidus temperature of a silicate glass that is formed from silica with one of the metal oxides alone. 3. The method according to claim 2, characterized in that the mixture has no nitrogen silicide, that the mixture between 15 and 45 wt.% Silica, between 0.05 and 50 wt.% Alumina and between 40 and 60 wt.% Aluminum nitride wherein the relative proportions of the components of the mixture are such that a ceramic material is formed at this temperature, the at least 95 wt.% Of a compound of formula SÌ6_zAl2N8_zOz contains, where z is greater than O and less than or equal to 5. 4. The method according to claim 2, characterized in that the mixture contains nitrogen silicide, that the relative proportions of the nitrogen silicide, alumina, aluminum nitride and silica in the mixture are such that a reactive composition is formed at said temperature which is at least 95 % By weight of the mixture mentioned, and in which the atomic ratio Silicon: aluminum: nitrogen: oxygen is 6-z: z: 8-z: z, where z is greater than zero and less than or equal to 4. 5. The method according to claim 2, characterized in that the mixture contains no nitrogen silicide and that the relative proportions of alumina, aluminum nitride and silica in the mixture are such that a reactive composition is formed at said temperature which is at least 95 wt.% of the mixture and in which the atomic ratio of Silicon: aluminum: nitrogen: oxygen is 6-z: z: 8-z: z, where z is greater than 4 and less than or equal to 5. 6. The method according to claim 2, characterized in that the metal oxides are selected from a group consisting of magnesium oxide, alumina, a manganese oxide, Lithium oxide, titanium dioxide, a boron oxide and iron oxide. 7. The method according to claim 2, characterized in that the metal oxides have magnesium oxide and trimangan tetra-oxide. 8. The method according to claim 2, characterized in that the metal oxides have magnesium oxide and manganese oxide in a weight ratio of 1: 4. 9. The method according to claim 2, characterized in that the metal oxides have magnesium oxide, manganese oxide and iron oxide. 10. The method according to claim 2, characterized in that the metal oxides have magnesium oxide and lithium oxide with approximately equal parts by weight. 11. The method according to claim 2, characterized in that the metal oxides have magnesium oxide and lithium oxide, which are present in a mixture with approximately equal proportions by weight, alumina and an oxide of boron also being present. 12. The method according to claim 2, characterized in that the metal oxides have magnesium oxide and titanium dioxide, which are present in the mixture in approximately the same proportion by weight. 13. The method according to claim 2, characterized in that the metal oxides comprise lithium oxide and alumina. 14. The method according to claim 2, characterized in that the amount of the metal oxides present in the mixture at the said temperature is such that the silicate glass is present in the sintered product up to 5% by weight. 15. The method according to claim 1, characterized in that the mixture contains a metal silicate. 16. The method according to claim 2, characterized in that the mixture has a lithium oxide and is free of manganese oxide. 17. The method according to claim 1, characterized in that the mixture contains silicon oxynitride. 18. The method according to claim 1, characterized in that the mixture contains aluminum oxynitride. 19. Sintered, ceramic product produced by the method according to claim 1.