WO1997003030A1

WO1997003030A1 - Process for fabricating an electrically insulating silicon carbide

Info

Publication number: WO1997003030A1
Application number: PCT/CH1996/000253
Authority: WO
Inventors: Dusko Maravic
Original assignee: Negawatt Gmbh
Priority date: 1995-07-13
Filing date: 1996-07-09
Publication date: 1997-01-30
Also published as: AU6186496A; JPH11508868A; EP0837837A1

Abstract

In a process for fabricating electrically insulating silicon carbide by means of liquid-phase sintering, wherein the sintering additives are added prior to sintering, the quantity of sintering additives, the sintering temperature and the duration of sintering are selected as process parameters. With the aid of these parameters, the quantity of intercrystalline phase remaining in the sintered silicon carbide is predetermined. This yields as end product a high-resistance electrical insulator.

Description

Process for the production of electrically insulating silicon carbide

The present invention relates to a method for the manufacture of electrically insulating silicon carbide by means of liquid phase sintering.

Sintered silicon carbide is a well-known industrial ceramic, which is used in a variety of ways due to its good mechanical and chemical resilience. Silicon carbide above all has good high-temperature resistance, thermal shock resistance, acid resistance and mechanical strength.

Silicon carbide ceramics can be produced by solid state sintering or by liquid phase sintering. Methods for the liquid phase sintering of silicon carbide have been known since the mid-eighties. In addition to the properties mentioned above, liquid phase sintered silicon carbide has a high fracture toughness, which corresponds approximately to that of silicon nitride.

Such a liquid phase sintering process is described, for example, in US Pat. No. 4,564,490. The powdered silicon carbide is mixed with metal oxides, called sintering additives. When the mixture is heated to the sintering temperature, a liquid phase forms and capillary forces cause the desired compression process. Usually used as sintering additives Aluminum oxide A1 ₂ 0 ₃ and yttrium oxide Y ₂ 0 ₃ up to 10 G% (weight percent) are used, the use of aluminum nitride A1N also being known. Furthermore, carbon C can be added as a further additive, which influences the wetting behavior between the liquid phase and the silicon carbide grains.

The production of sintered silicon carbide by means of the liquid phase has the advantage that ceramics with preferred mechanical or thermomechanical properties can be produced by adding certain sintering additives. In this case, only a small amount of sintering additives is always added, there being a lower limit, the undershoot of which results in an insufficient sintering behavior.

The amounts of sintering additives added were chosen so that the desired mechanical properties could be achieved. A maximum amount of sintering additives was assumed, the exceeding of which no longer leads to an improvement in the properties, but rather deteriorates the high-temperature and corrosion properties.

The electrical properties of the end product were not specifically optimized; on the contrary, they were generally not taken into account. Pure silicon carbide is an electrical insulator. However, pure silicon carbide is hardly produced, since raw materials are used which are not contaminated. These impurities considerably reduce the electrical resistance. In addition, the addition of the known sintering additives leads to a further reduction in the electrical resistance, since the silicon crystals are doped with the sintering additives. The known ceramics made of liquid sintered silicon carbide are therefore semiconductors. The The specific resistance achieved is primarily determined by the type and quantity of the added sintering additives. The influence of the sintering additives on the electrical properties is also not unlimited, but is determined by the solubility of the additives in silicon carbide.

In the journal Am. Ceram. Soc, 70 [10] C 266-267, Y. Takeda et al. , sintered silicon carbide is described, which is an electrical insulator. Beryllium oxide is used as a sintering additive. The isolating effect is caused by the beryllium or beryllium oxide dissolved in the silicon crystals. However, beryllium oxide is highly toxic and therefore unsuitable for most ceramics applications.

Another sintering additive which leads to an electrically insulating ceramic with a specific resistance of 10 ⁸ ohm cm at room temperature is known from US-A-4'701'427. Boron B is used here as a sintering additive. In addition, carbon C is added or the sintering process is carried out in a nitrogen atmosphere. The temperature required to achieve the desired result is between 2250 ° C and 2350 ° C. In this process, the silicon carbide crystals are doped with nitrogen, as a result of which an increase in the density of the product is achieved. It was found that the end product must have a density of at least 95% of the theoretical density of pure silicon carbide in order to obtain an electrical insulator. This is the only way to ensure that enough nitrogen has been stored to achieve an electrically insulating effect. This process is also subject to saturation. In addition, as with the other known methods, relatively high temperatures have to be used, so that the production costs are high. It is therefore an object of the invention to provide an alternative method for producing electrically insulating silicon carbide.

This object is achieved by a process for the production of sintered silicon carbide by means of a liquid phase, sintering additives being added before the sintering, which is characterized in that the quantity of sintering additives added, the sintering temperature and the sintering time period are selected as process parameters and by means of these The quantity of the grain boundary phase remaining in the sintered silicon carbide is predefined.

The liquid sintering method according to the invention enables the production of ceramics from silicon carbide with specific electrical resistances in the range of greater than 10 ¹³ ohm cm at room temperature.

The main difference from the known methods is that the solubility of the sintering additives in silicon carbide is not essential, but that the process parameters are selected so that a certain amount of remaining grain boundary phase is retained in the end product. The sintering additives dissolved in the crystal reduce the electrical resistance of the individual grain. However, the remaining grain boundary phase increases the overall electrical resistance of the silicon carbide body.

The silicon carbide produced by the method according to the invention does not necessarily have the same good mechanical and chemical properties as the known ceramics made of silicon carbide. For most application areas, however, they are still sufficient. The Swiss application CH 02 854 / 94-1 describes possible applications in the medium temperature range.

In a preferred variant of the process, aluminum oxide A1 ₂ 0 ₃ and yttrium oxide Y _: 0 ₃ are used as sintering additives. In order to obtain high-resistance silicon carbide, at least 8 V% (volume percent) grain boundary phase must be present when using these sintering additives after the sintering process. If less than 5 V% is present, the specific electrical resistance is less than 10 "ohm cm. The amount of the added sintering additives must therefore be chosen so large that, despite the weight loss during sintering, a sufficient amount of grain boundary phase remains in the ceramic , which ensures a complete coating of all silicon carbide grains.

In contrast, attempts have been made in the known processes to keep the content of sintering additives in the end product as low as possible in order to obtain silicon carbide which is as pure as possible. The sintering additives only served to improve the sintering behavior or basically to effect a sintering process.

Another advantage of the method according to the invention is that it is possible to work in a lower temperature range, preferably 1800 ° C.-1900 ° C. This significantly reduces manufacturing costs.

The use of other intermediate additives and / or other mixing ratios is possible. As a result, the minimum required residual amount of grain boundary phase and / or the required sintering temperature and sintering period can also change. The enclosed graphical representations serve to explain the method according to the invention. Show it

Figure 1 shows the theoretical density of the sintered body as a function of weight loss in the sintering process;

FIG. 2 shows the specific electrical resistance as a function of the amount of the grain boundary phase;

Figure 3 is a schematic representation of the grain boundary phases of a sintered body according to the invention and.

Figure 4 is a schematic representation of the grain boundary phases of a sintered body according to the prior art.

Some preferred variants of the method according to the invention are described below:

Silicon carbide SiC in powder form is used as the starting material. The silicon carbide powder preferably has a large surface area, greater than 10 m ² / g. In this example, a powder with an average grain diameter of 0.7 my is used.

Aluminum oxide Al-O3 and yttrium oxide Y ₂ 0 ₃ are added to this silicon carbide powder as sintering additives. Silicon oxide Si0 _{2 is} present as a further component, which is always present on the surface of fine-grained or powdery silicon carbide SiC and is therefore not referred to as an additive. These three components A1 ₂ 0 ₃ , Y ₂ 0 ₃ and Si0 ₂ form the liquid or grain boundary phase in the sintering process. The sintered mixture is mixed and homogenized using known methods, for example in an attritor with an aqueous emulsion which has a solids content of approximately 50% by weight and a proportion of 2% by weight of organic substances. The mixture is then dried and pressed into a desired shape. In the test series described here, cylindrical tablets with a diameter of 20 mm and a height of 5-7 mm were formed under a uniaxial pressure of 150 MPa. The density of the blanks is determined.

These blanks are then sintered. The sintering process can be carried out without pressure by heating in an oven or by means of hot presses. Argon is preferably used as the atmosphere. The sintering temperature and the sintering period essentially determine the weight loss. Both parameters are selected so that at least 8 V% grain boundary phase remains after sintering.

In a first example, the following sintering additives were added:

6 G% aluminum oxide A1 ₂ 0 ₃ (G% = weight percent) 4 G% yttrium oxide Y ₂ 0 ₃ .

As a further additive, 2 G% silicon oxide Si0 _{2 is} present, which are already present on the surface of the silicon carbide powder.

The liquid or grain boundary phase formed by these three components thus comprises 12 G% of the sinter mixture.

The sintering process takes place using hot presses at a temperature of 1800 ° C. and an applied pressure of 30 MPa, which is invested for a period of approximately one hour. The measured weight loss is 2.8 G%, so that 9.2 G% (this corresponds to 8 V%) of the grain boundary phase remains in the end product. The end product has a specific electrical resistance of greater than 10 ^{± 3} ohm cm. Furthermore, the measured 4-point bending strength at room temperature is greater than 400 MPa, at 1400 ° C it is still 300 MPa. The fracture toughness is 6 MPa m ^1/2 and the thermal conductivity at room temperature is 70 W / mK and at 1200 ° C still 30 W / mK.

In a second example, aluminum oxide A1 ₂ 0 ₃ and yttrium oxide Y ₂ 0 _{3 are added} in other amounts, namely 7.2 G ^& A1 ₂ 0 ₃ and 4.8 G% Y ₂ 0 ₃ , so that a total of 14 G% additives are contained in the sintered mixture that form the liquid phase. The sintering process takes place in an argon atmosphere at normal pressure. The blanks are heated in a furnace to a temperature of 1800 ° C - 1900 ° C, the heating rate being 10 ° C / min. This maximum temperature is maintained for a certain period of time. At a heating temperature of 1875 ° C, which is present for a period of one hour, the measured weight loss is 4.5 G%, so that 9.5 G% of the grain boundary phase remains in the end product.

The theoretical density of the final product is not a constant but a function of the composition of the starting materials and the inorganic weight loss during the baking process. This function is shown in FIG. 1 for a mixture of 90 G% silicon carbide SiC, 6 G% aluminum oxide Al ₂ 0 ₃ and 4 G% yttrium oxide Y ₂ 0 ₃ . If there is no weight loss, the end product will have the same Density like the mixture before sintering, namely 3.296 g / cm ³ . When the liquid or grain boundary phase is completely evaporated, the density of the pure silicon carbide SiC is still 3.21 g / cm ³ . It has been shown that the maximum density and thus the minimum weight loss is obtained with a heating time of approximately one hour. The weight loss increases linearly as a function of the heating period. In order to obtain an electrical insulator with the desired good mechanical and thermomechanical properties, the period of time for the heating must therefore be optimized. If the bake is too short, the sintering process is not complete and the end product does not have the desired mechanical or chemical resistance. If the heating is too long, however, too many sintering additives are evaporated, so that the end product becomes an electrical semiconductor or conductor.

FIG. 2 shows a graphical representation of the specific resistance as a function of the volume content of the sintering additives mentioned. The reference number A denotes pure silicon carbide SiC. The range from A to B shows the specific resistance value for impurities due to the raw material, B to C the doping of the SiC crystal lattice due to the sintering additive. C denotes the maximum solubility of the sintering additives in silicon carbide. The grain boundary phase fractions increase in the area C to D, the individual SiC grains still being incompletely coated. At point D almost all SiC grains are coated. From point D to E, the grain boundary phase fractions increase, but this increase no longer affects the electrical resistance.

It can be seen from FIG. 2 that with a content of less than 5 V% of sintering additives, the end product is no longer an electrical insulator. The following explanation is possible: for one Grain boundary phase fraction of 8 V%, all silicon carbide grains are almost completely surrounded by grain boundary phases, but only partially at 5 V%. Measurements have shown that the average thickness of the grain boundary phase involved with 9.2 G% is around 30 nm and the average distance between the two SiC grains is in the range of 60 nm.

A silicon carbide produced according to example 1 is shown in FIG. 3. The grain boundary phase has a low surface tension and therefore a small wetting angle and good wetting. This results in an at least approximately complete encapsulation of all SiC grains by the three-component liquid phase melt D. The electrically conductive SiC grains remain completely enveloped by the grain boundary phase even after the sintered body has cooled, so that a high-resistance electrical insulator is produced.

In a sintering process according to the prior art, however, carbon C is added as a further sintering additive. Such a sintered body is shown in Figure 4. This admixture reduces or completely eliminates the Si0 ₂ present on the surface. The two-component liquid phase melt Z, consisting of A1 ₂ 0 ₃ and Y ₂ 0- ,, has a high surface tension and thus a large wetting angle and poor wetting. It does not allow complete encapsulation of all SiC grains. When the sintered body cools, the liquid phase predominantly retracts into the triple points T, that is to say into the largest gaps between the SiC grains. A significant proportion of the SiC grains remain in direct contact with one another, so that a SiC body sintered in this way is not an electrical insulator.

Claims

claims

1. A process for the production of electrically insulating silicon carbide by means of liquid phase sintering, sintering additives being added to a silicon carbide powder prior to sintering, characterized in that the amount of sintering additives added, the sintering temperature and the sintering time period are selected as process parameters and by means of these parameters the amount of the grain boundary phase remaining in the sintered silicon carbide is predefined.

2. The method according to claim 1, characterized in that such an amount of grain boundary phase is retained in the sintered silicon carbide that at least approximately all of the silicon carbide grains contained in the sintered silicon carbide are completely encased.

3. The method according to claim 1, characterized in that aluminum oxide A1 ₂ 0 ₃ and yttrium oxide Y ₂ 0 ₃ are used as sintering additives and that a three-component grain boundary phase is formed with the silicon oxide Si0 ₂ present on the surface of the silicon carbide powder.

4. The method according to claim 3, characterized in that the amount of grain boundary phase remaining in the sintered silicon carbide is at least 8 percent by volume.

5. The method according to claim 3, characterized in that approximately 6 weight percent aluminum oxide A1 ₂ 0 ₃ and approximately 4 weight percent yttrium oxide Y ₂ 0 _{3 is added} as a sintering additive.

6. The method according to claim 3, characterized in that approximately 7.2 weight percent aluminum oxide Al ₂ 0 ₃ and approximately 4.8 weight percent yttrium oxide Y ₂ 0 _{3 are added} as sintering additives.

7. The method according to claim 1 or 6, characterized in that the sintering temperature is 1800 ° C - 1900 ° C and the sintering time period is approximately one hour.

8. The method according to claim 1, characterized in that sintering in an argon atmosphere.

9. The method according to claim 1, characterized in that sintered without pressure or by means of hot presses.

10. Sintered silicon carbide, produced by the method according to any one of claims 1-9, characterized in that the amount of the grain boundary phase remaining in the sintered silicon carbide is sufficient to completely encase at least approximately all of the silicon carbide grains contained in the sintered silicon carbide.

11. Sintered silicon carbide, produced by the method according to one of claims 1-9, characterized in that at least 8 percent by volume of grain boundary phase is present.