DE2511650C2 - Process for making alumina whiskers - Google Patents

Description

The invention relates to a method for producing aluminum oxide whiskers according to the preamble of the claim.

In known processes for the production of aluminum oxide whiskers whiskers are produced by the controlled oxidation of liquid aluminum, whereby a mixture of oxygen and a gas that does not react with the aluminum over the surface of liquid aluminum is passed. Here, the aluminum oxide whiskers are on the surface of the liquid Aluminum formed, but they are contaminated by small aluminum balls.

Another proposal for the formation of aluminum oxide whiskers is the oxidation of gaseous ones Aluminlum-Trlchlorld with carbon dioxide in the presence of sufficient hydrogen to establish a connection with the chlorine present in the aluminum trichloride and to form hydrogen chloride. (Campbell, W. B. - Determination of the Feasibility of Forming Refractory Fibers by a Continuous Process. Lexington Laboratories Report No. AMRA CR-63-03 / 8F - 1966.)

The reaction will take place at temperatures between 1275 and 1700 ° C. The physical form of the whisker depends on the degree of supersaturation of the reacting system with the generated aluminum oxide; she is however, it is not easy to control and it is not easy. To produce whiskers from aluminum oxide with the substantial elimination of other forms.

The object of the invention is to improve of a process for the production of aluminum oxide whiskers according to the type cited in the introduction. in which the whiskers can be quickly freed from inclusions and a generation of the whiskers in desired, easily controllable shape allows.

The solution to the problem is based on the method mentioned from and is indicated in the identifier of the claim.

The aluminum oxide dials produced in this way contain no inclusions, and further, the whiskers can be easily controllably produced in the desired shape will.

The method according to the invention depends on the ablaut the following reaction:

3C + ■ AIX ₁

3ALX + 3CO

where X the halogen is chlorine or bromine. The course of such a reaction has been established on the basis of thermal conditions; however, experimental evidence for this has so far only been carried out to a limited extent. It has now been found that this reaction not only takes place at higher temperatures of the order 1400 to 1800 ⁰ C, but that, so that the temperature is decreasing the temperature to about 200 to 400 ° C under 1400 ° C, the reverse The reaction takes place with the deposition of aluminum oxide whiskers and the precipitation of carbon.

Because of the freedom from inclusions, the corresponding anhydrous, gaseous aluminum trihalogen formed in a pre-reaction zone by adding a vapor mixture of the appropriate halogen gas and an inert Carrier gas is passed over a bath of liquid aluminum of high purity. The carrier gas can from inert gases of the atmosphere existing, z. B. of helium or argon or a mixture thereof. One to Recommended reaction temperature is about 700 ° C, experiments have shown that this temperature is below the temperature at which the aluminum monohalides are present. The resulting The mixture from this reaction is then poured over a heated, unmelted body of aluminum which has a large surface area in relation to its volume, e.g. B. in the form of an or multiple coils to ensure that no unreacted Halogen is present in the gas mixture which is passed at the first reaction time.

In the first reaction zone, the aluminum trichloride vapor or aluminum bromide vapor, to which inert gas is admixed, is used. Passed over one or more layers of an intimate mixture of finely divided aluminum oxide and carbon. The mixture has stoichiometric ratios. as required according to the above equation. Dis temperature in the first Rcaktlonszonc is 1400-1800 ° C, preferably 1500-1700 ⁰ C, with the higher temperatures are used by the first reaction zone with faster flow rates of the Rcaktandcn. The flow rate of the inert gas through the reaction zones and the concentration of aluminum chloride or aluminum tribromide therein should be such as to ensure that the inert gas is slightly supersaturated with aluminum oxide under the prevailing conditions in order to create conditions which are conducive to the deposition of aluminum oxide in whisker form Is, namely after the reaction with the aluminum oxide and the carbon with the occurring formation of aluminum chloride or aluminum bromide and subsequent cooling in the second reaction zone with the reversal of the reaction that took place in the first reaction zone. When the inert gas becomes too supersaturated, a mixture of electrolyte and aluminum oxide platelets begins to precipitate, and the greater the degree of supersaturation, the greater the ratio of platelets deposited. The platelets can be so small that they resemble powder form with very substantial degrees of supersaturation, which should be avoided.

The optical properties of the alumina whiskers produced reveal that these single crystals form and that when other single crystals grow Best results are obtained when the crystals are In grow in a slightly saturated medium. It is one A plurality of variables are given, and with each existing device standard, some become preceding ones Experiments may be necessary to find the optimal conditions

to find out in each individual case.

Therefore, when performing previous experiments the production of a mixture of whiskers and flakes from aluminum oxide, in which flakes or powdery aluminum oxide predominate, indicate that in the cooler second reaction zone under the conditions of the experiment too great a degree of supersaturation of the entering the cooler zone Gas flow has occurred. Such occurrence must be prevented, e.g. B. either by reduction the halogen ratio in the gas stream contacting the molten aluminum or by slower Cooling of the media flowing out of the first zone, in which the aluminum trichloride vapor or aluminum / methyl bromide vapor is in contact with the intimate mixture of aluminum oxide and carbon, or by reducing the absolute pressure in the gas stream, or some combination of these Measures.

Graphite in particular is used as carbon.

In the second, cooler reaction zone, the reaction proceeds in reverse, ie from right to left in relation to the above equation, and results in the formation of aluminum oxide and carbon. Preferably, the alumina is allowed to condense on the surface of vines substrates, e.g. B. a boat made of alpha alumina to knock down with the substrate in the second zone. The carbon is deposited in a zone near the second zone where the alumina whiskers precipitate, the zones usually overlapping. It is then necessary to remove the carbon from the aluminum oxide electrolyte by interrupting the supply of the gas flow containing the aluminum trhalogen and replacing this flow with an oxygen-containing gas flow, the other components of which are related to each other a raised Tcmperaturpegcl located aluminum oxide are inert, z. B. nitrogen. The temperature should be maintained at such a level that the carbon present is oxidized to carbon monoxide and / or carbon dioxide. Temperatures of 800 ° C and above are suitable for this, e.g. B. 800 to 1000 ⁰ C. This is converted catfish of the contaminating carbon in gaseous Kohlcnstoffmonoxyd and / or carbon dioxide, and these gases are removed with the saucrstoffhaltlgcn gas stream.

The second, cooler reaction zone is a zone with decreasing temperature in the direction of flow of the reaction mixture. The temperature of this zone can therefore fall of 1400 ⁰ C at its one end which is closer to the first Rcaktlonszonc to 1000 ⁰ C at the other end.

When the reaction takes place from left to right according to the above equation, it is accompanied by an increase in volume the formation of the reaction products; accordingly there is a decrease in volume when the reaction runs from right to left. However, since it is important that the Rcakilon be in the first Rcaktlonszonc from the left runs to the right, it is advantageous that a sub-atmospheric cr Pressure is maintained in the first reactor zone. Since it is also most expedient, with a continuous Flow from the first reaction zone to the second reaction zone to work, these zones Maintained under substantially the same pressure. Is it advantageous to place both zones on one sub-atmospheric pressure Hold pressure. Preferably a Pressure from 0.05 to 0.50 atmospheric pressure is applied.

The behavior of the alpha-aluminum oxide voters, viewed in a beam of polarized light indicates that the whiskers are C-type, the grew as individual crystals on the 0001 axis (according to Miller indices). This type exhibits a regular Hexagonal cross-section. Samples examined by electron microscopy had an extremely high aspect ratio, namely 100 to 10,000 to 1.

A test sample is described in detail below.

A batch was prepared consisting of an intimate mixture of three parts by weight of aluminum oxide and a weight fraction of graphite.

Part of the batch was used to fill a container with recrystallized alpha alumina? .U, the container being housed in a tubular electric furnace. One end of the oven came with a Device for producing anhydrous aluminum trichloride connected by adding chlorine gas over liquid Aluminum was passed, while the other end was connected to a condenser with a discharge line was. The apparatus for producing anhydrous aluminum trichloride was also provided with a supply source but connected to gas, in this case argon. There was also a vacuum pump with the device connected so that the entire device could be operated at reduced pressure.

The device equipped in this way was first blown out with argon in order to remove oxygen and moisture. The aluminum metal was then melted while the argon continued to be blown through. After the aluminum had completely melted, chlorine was added to the argon in a preliminary section and the mixture was passed over the liquid aluminum. A reaction took place on the surface of the aluminum, during which the anhydrous ?: aluminum trichloride was formed, which after its formation evaporated and was carried away by the argon flow. The resulting gas stream of aluminum trichloride was then passed at 1000 ⁰ C by a preheater and then introduced into the tubular electric furnace, which was operated at 1600 ° C, where the gas stream in the intimate mixture of alumina and graphite diffusely invaded and thus reacted to to cause the formation of aluminum chloride and carbon oxide.

Except for the aluminum oxide container, the temperature fell gradually decreasing. In the part of the furnace tube that was in the reduced temperature zone, was produced a woolly mass of aluminum oxide electrolyte core, and a precipitate formed in close proximity to it of carbon contaminating an area of the woolly mass. It became a pressure of 0.25 absolute Atmospheric pressure driven.

The gases emanating from the furnace were cooled and through a condenser operated at 200 ° C directed. The liquid aluminum was collected in a swamp and solidified there. The residual gases were washed with alkali and then into the atmosphere derived.

The contents of the container or boat were cleaned of carbon in a separate operation. By doing a mixture of oxygen and nitrogen via a Shell made of aluminum oxide was passed. In which the mixture of aluminum oxide selector core and contaminating Carbon was placed. The shell was heated in an oven to approximately 1000 "C until all

Carbon was oxidized.

Further experiments were carried out at temperatures in the range from 1500 ^° C. to 1700 ^° C. and at pressures from 0.1 to 1.0 atmospheric pressure. Whiskers were obtained when the partial pressure of argon was greater than that of the anhydrous aluminum chloride. The partial pressure should not be greater than 20 * of the total pressure in the reaction zone.

Typical samples of the alumina whiskers produced by the above method, each having a thickness

greater than one micrometer were subjected to various mechanical tests including strength tests subjected. The following results were obtained:

Average Elongation at Break: 1.20% Average Tensile Strength: 5.52 GH / m ² RMS Diameter 2.86 micrometers True Resilience
(Young's modulus E): 460.9 GN / m ²

Claims (1)

Claim: Process for making aluminum oxide whiskers by contacting an anhydrous aluminum trichloride or aluminum tribromide vapor with an intimate mixture of carbon and aluminum oxide in between 1400 and 1800 ° C held first reaction zone, in which a mixture of carbon monoxide and aluminum trichloride or aluminum tribromide, characterized in that said Mixture to a second reaction zone with a 200 to 400 ° C lower, but below 1400 ° C lying temperature is passed.