Self-supporting ceramic body and process for preparing thereof

The invention generally relates to self-supporting ceramic bodies and to a process for their production, in particular composite ceramic bodies.

Methods for producing self-supporting ceramic bodies using the oxidation phenomenon of a metal precursor have been described in the patent literature. Oxidation reactions can be promoted by the use of a dopant alloying the parent metal, which allows the production of self-supporting ceramic bodies of the desired size, grown as the parent metal oxidation reaction product.

The oxidation process has been further improved by using external dopants which are applied to the surface of the metal precursor. Methods for producing self-supporting ceramic bodies enclosing one or more fillers in a matrix have been described in the patent literature. In this method, the oxidation reaction product is allowed to grow from the parent metal into the permeable filler mass. However, the resulting composite has neither a defined nor a predetermined shape.

The possibility of creating a defined or predetermined shape of the ceramic body, i.e. the possibility of growing the ceramic body to a predetermined shape and dimension, was achieved by allowing the oxidation reaction product to infiltrate into the shaped body from the preform to its surface area.

Further development of said methods makes it possible to produce self-supporting ceramic structures comprising one or more cavities that negatively copy the shape of the positive body from a shaped base metal precursor embedded in a bed of permeable adaptive filler which is at least partially self-binding under specified conditions. Yet a further development of said methods makes it possible to produce self-supporting ceramic bodies having a shape that negatively follows the positive shape of the base metal precursor placed against the fill mass. In both these cases, the cavity negatively follows the shape of the parent metal.

For certain applications, there is an increasing interest in replacing metals with ceramic materials because certain properties of ceramics are superior to those of metal. However, there are several limitations or problems with such replacement, such as shape and size adaptability, the possibility of creating complex shapes, achieving the properties required for the purpose of use, and cost.

Also described in the patent literature are methods for producing ceramic bodies having complex internal cavities, particularly inwardly extending cavities. Conventional or known powder compression and sintering processes are not useful in the manufacture of ceramic products of this shape, since the inner core, which is necessary to form the inner cavity of the ceramic body, is practically capable of being removed after the body has formed around it. Although parts with such geometric shapes can be made by machining the ceramic product to the desired shape, this method is rarely used because it is too expensive.

The present invention provides a self-supporting ceramic body comprising a polycrystalline matrix from a product of the oxidation reaction product of a molten parent metal selected from the group consisting of aluminum, zirconium, titanium, tin and silicon, and an oxidant closing or filling the optional filler. SUMMARY OF THE INVENTION The body of the invention comprises one or more channels which form continuous paths, the inlet and outlet ends of which are accessible from the surface of the body, and which follow the shape of a volatile metal formation originally located in the growth path of the oxidation reaction product. The metal is dispersed in a polycrystalline matrix. Suitably, the channel walls are coated.

The present invention also relates to a process for producing said ceramic body by oxidizing the molten parent metal with an oxidant to produce an oxidation reaction product in the form of a polycrystalline matrix, optionally enclosing the filler. The essence of the process of the present invention is to provide a base metal body with a volatile metal body corresponding to the channels in the ceramic body to be produced, and to heat the system above the base metal melting point and below the oxidation reaction product melting point. the volatile metal dissipates with the formation of channels. To form a composite ceramic body, the parent metal and the volatile metal formation are inserted into the permeable filler. According to the invention, the volatile metal used is aluminum, iron, nickel, chromium, alloys thereof and intermetallic compounds or mixtures thereof. Suitably, a coating, in particular chromium trioxide and / or nickel oxide, is applied to the volatile metal, for example formed from wires. The coating prevents premature dissolution or premature collapse due to melting, whereby the volatile metal would lose its predetermined shape. In addition, a coating can be chosen such that, when the volatile metal is dispersed, it forms a layer on the channel walls which emphasizes the properties of the ceramic body. The channels enclosed in the ceramic body according to the invention are made accessible by mechanical treatment of at least one outer surface of the body.

The method according to the invention makes it possible to produce ceramic bodies comprising one or more channels inside, including complex channels which are interconnected, have converging directions, acute angles and a complex shape, faithfully negatively following the approximate dimensions and configuration of the formed volatile metal. The desired geometry of the internal openings and channels in the ceramic body can be achieved much more easily than by conventional methods of drilling, grinding, or the like of the ceramic non-machined body.

In the following, the following terms are to be understood as follows: Ceramics is not limited to a ceramic body in the classical sense, that is to say a body consisting exclusively of non-metallic and inorganic materials, but refers to a body which is predominantly ceramic either in composition or in principal properties, although the body may contain less or more or a plurality of metal constituents derived from a parent metal or reduced from an oxidant or dopant; this content is typically 1 to 40% by volume, but the metal content may be even greater.

Oxidation reaction product: generally means one or more metals in an oxidized state where the metal has given up electrons or shares electrons with another element, compound or combination thereof. Thus, the oxidation reaction product comprises the product of the reaction of one or more metals with an oxidant.

Oxidant means one or more electron acceptors or electron-sharing substances and can be in a solid, liquid or gaseous state or a combination thereof, for example in a solid and gaseous state, under the process conditions of the invention.

Base metal means that metal, for example, aluminum, which is a precursor of a polycrystalline oxidation reaction product, and includes the metal as a relatively pure metal, as a commercial metal with impurities and / or alloying impurities, or an alloy in which the metal precursor is the major component. Wherever ae refers to a particular metal, such as aluminum, as the parent metal, this is to be understood in the sense of this definition.

Volatile metal means a metal, intermetallic compound or alloy which, after being surrounded by a growing oxidation reaction product, is dispersed in a polycrystalline material leaving a channel that is substantially the same in shape and size as the space previously occupied by the volatile metal . Of course, the volatile metal may be of any desired or useful configuration, shape or form, such as hollow bodies, particles, powders, fibers, wires, balls, bubbles, metal wool, plates, aggregates, rods, sticks, pellets, tubes, wire cloth, sponge structure, tubes and sheets.

A channel or channels means an unfilled space, a cavity, a passage, and the like, and may not have uniform dimensions. This passage is formed within a mass or body of a suitable or desired shape and is not limited to a tubular configuration.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be explained with reference to the drawings, in which: FIG. 1 is a cross-sectional view of a system consisting of a precursor metal and a fugitive metal formed side by side in a bed of granular filler; Fig. 3 is a cross-sectional view taken along line 3-3 of Fig. 1 in a slightly enlarged scale of the assembly of Fig. 1, with the filler omitted for clarity; 4 Fig. 5, 6 and 7 are photomicrographs of cross-sections of ceramic composite materials produced according to Examples 1, 2 and 3, respectively, in an enlarged perspective view of a self-supporting ceramic composite produced by the assembly of Fig. 1; 3 and 8 show a photograph of a composite ceramic body provided with channels produced by the method of the invention, one of the channels being illuminated from behind to show that it is continuous.

In carrying out the process of the invention, the parent metal forms part of a system consisting of a parent metal and a shaped volatile metal. The volatile metal is formed into a configuration to be negatively copied by one or more channels within the ceramic body, in particular the ceramic composite body. By the method according to the invention, it is possible to produce complex shapes and channel systems in the ceramic body which negatively follow the original shape of the volatile metal. The term negative copying means that the channels in the ceramic body are defined by the inner surfaces of the ceramic body that correspond to the shape of the molded volatile metal used. The shaped volatile metal may be suitably shaped by any method or means: for example, a metal wire may be formed into a system that indicates the desired shape, size and location of the channel system. Alternatively, a piece of metal such as a rod, foil, tube or plate may be suitably machined or drawn to the desired shape, or the shaped volatile metal may be cast, pressed, extruded or otherwise shaped to match the geometry of the channels to be formed. in a ceramic composite body. Typically, the length dimensions of the shaped volatile metal elements are greater than their cross-section. This makes it possible to produce ceramic bodies with very narrow or fine channels or openings, for example by using wires as a shaped volatile metal. The shaped volatile metal may consist of one or more pieces of metal that may be suitably shaped in any way so that when placed in the filler bed or adjacent to the parent metal precursor, the growing polycrystalline material formed by oxidation of the parent metal surrounds the shaped volatile metal and possibly infiltrates or at least a portion thereof.

The materials and reaction conditions are selected such that the polycrystalline material does not grow into space and thus occupy the space that originally occupied the shaped volatile metal, but to surround the shaped volatile metal, which is then dispersed in the surrounding polycrystalline material. AND? the volatile metal dissolves or diffuses into the parent metal or alloy or otherwise reacts with the polycrystalline material or its constituents, the result being that the shaped volatile metal eventually migrates from the space it originally occupied to the body of the polycrystalline material surrounding it. This creates a shaped channel that negatively copies the original geometry of the shaped volatile metal. Thus, a great advantage of the method according to the invention is that the geometry of the channels to be formed in the ceramic body can be determined by shaping or machining the shaped volatile metal instead of drilling or otherwise machining the ceramic body.

The base metal precursor may have any suitable or appropriate shape, for example, it may be an ingot, plate, rod or the like that forms the source of the base metal. If the desired channels are formed, the parent metal need not have a particular shape or configuration if enough of the parent metal is positioned relative to the shaped volatile metal to surround all or part of it. The parent metal may be selected from the group consisting of aluminum, zirconium, titanium, tin and silicon.

The filler which may optionally be used according to the invention may be one or more of a variety of materials suitable for this purpose. The filler may consist of particles of material such as fine grains of refractory metal oxide, for example alumina, or may be in the form of fibers or wires or in the form of a fibrous material in the form of wool, for example ceramic fibers. The filler may consist of a combination of two or more such geometric shapes, for example a combination of small grains and fibers. It is only necessary that the body arrangement of the filler allows placement of the formed volatile metal on or into the bed of filler, and that the filler permits the oxidant under oxidation reaction conditions when its passage is necessary to contact the oxidant with the parent metal and reaction. If a gaseous oxidant is used, the filler must be permeable to the oxidant to allow it to contact the molten parent metal within its mass.

In carrying out the process according to the invention, the system consisting of a shaped volatile metal and a base metal precursor is arranged by placing the shaped volatile metal and the base metal side by side. These components of the assembly may be supported in any suitable manner, for example, by supporting the formed fugitive metal entirely or in part with the filler bed. The filler bed may also be in contact or may partially or completely surround the parent metal body. The filler is not obligatory, so that the fugitive metal and the parent metal can be stacked side by side without the use of fillers.

In both cases, the system is heated to a temperature ranging above the melting point of the molten parent metal and ending below the melting point of the oxidation reaction product. This heating produces a body or bath of molten parent metal, which is subjected to an oxidizing environment at a temperature in the range indicated. The molten parent metal reacts with the oxidant to form a polycrystalline material consisting of the oxidation reaction product, starting to form the volatile metal in the growing polycrystalline material. At least a portion of the oxidation reaction product is maintained in contact with and between the molten parent metal and the oxidant such that upon further oxidant treatment, the molten parent metal is successively drawn into the oxidation reaction product and the oxidant, so that at the interface between the previously formed oxidant. the oxidation reaction product and the oxidant continue to grow another oxidation reaction product. The growing oxidation reaction product infiltrates the filler when the filler is used and surrounds the formed volatile metal. The reaction is maintained until a growing oxidation reaction product in the form of a polycrystalline material has surrounded the shaped volatile metal or a selected portion thereof, which is then dispersed in the polycrystalline material.

The polycrystalline material of the ceramic body produced by the process of the invention may contain, in addition to any filler enclosed therein, one or more metal constituents, for example unoxidized constituents of the parent metal, or may contain cavities or both, depending on the oxidation conditions reaction. The polycrystalline material also contains dispersed components of the volatile metal at least in the vicinity of the space originally occupied by the shaped volatile metal. In polycrystalline materials, the oxidation reaction product typically contains interconnected crystals, particularly in three dimensions. The metal components, inclusions or voids may also be at least partially interconnected.

The volatile metal may include those metals whose melting point is approximately equal to or lower than the growth temperature of the oxidation reaction product, but in these cases the fidelity of the volatile metal shape reproduction is less perfect due to deformation caused by premature melting or softening of the shaped volatile metal under oxidation reaction conditions. . The adverse effects of such premature melting or softening on the fidelity of negative reproduction can be completely eliminated or at least attenuated by supporting the shape of the volatile metal with a bed or by coating the metal with a refractory coating. For example, the shaped volatile metal may have a coating of fine particles of a self-bonding or sinterable material so that at elevated temperature the coating binds or sintered to form a hard shell that encloses the shaped volatile metal. The coating or shell should be impermeable to prevent the volatile metal from being dispersed into the oxidation reaction product. The coating may be of a type that reacts, disperses or is indistinguishable from the polycrystalline material.

The problem of premature softening or melting of the formed volatile metal and thus loss of fidelity in the negative copying of the formed volatile metal can be solved by using a metal or alloy whose volatile temperature is above the oxidation reaction temperature. When aluminum parent metal is used, suitable volatile metals whose melting point is above that of the oxidation reaction product include, for example, iron, chromium and aluminum alloys.

In some cases, it may be desirable to coat the channel walls to alter or improve the properties of the walls. The coating may be formed by coating the molded metal with a suitable material, which is then absorbed by the ceramic body and becomes part thereof in the region of the formed channels. Alternatively, a coating material may be selected that reacts with the molten parent metal to form a compound, for example an oxide, which then forms a coating of the channels. Under oxidation reaction conditions, the volatile metal that carries the coating is dispersed into the polycrystalline material and a coating is formed from the coating material in the region adjacent to the channels. The coating material is selected such that the coating has the desired properties, such as corrosion resistance. For example, the shaped volatile metal may be coated with non-reactive particles such as silicon carbide, alumina and the like. The growing polycrystalline material grows between the particles and absorbs the volatile metal, thereby forming in the channels a coating consisting of a matrix of polycrystalline material in which these non-reactive particles are enclosed. Alternatively, the shaped volatile metal may be coated with a reactive compound such as a metal which can be reduced by metal. The coating is applied, for example, by forming a paste of oxide particles and a suitable binder, such as an organic adhesive, and applying one or more layers to the formed volatile metal to give the coating a desired thickness. * In the manufacture of polycrystalline material from tZ-alumina by oxidizing the aluminum parent metal in the air, an oxide such as chromium oxide which is reducible by molten aluminum is applied in the form of a coating on a volatile metal such as an iron alloy, and aluminum called Kanthal.

Apparently, chromium trioxide is reduced to metallic chromium, which is dispersed in the ceramic body, probably by reaction with one or more metals of aluminum as the parent metal. Oxidation taking place simultaneously with the reduction of chromium oxide produces alumina, with the result that the channel walls in the ceramic body are substantially coated with alumina. Such techniques can also be used in conjunction with one or more inert granular materials, for example by mixing the chromium trioxide and alumina particles and applying a coating from the mixture to the volatile metal. Thus, it is apparent that the coating applied to the shaped fugitive metal serves not only to form the coating on the channel walls, but also to introduce one or more components which are either as such or as reaction product components dispersed in the ceramic body and not limited to the walls of the channels.

Fig. 1 shows a refractory vessel 2, for example of alumina, which comprises a bed of filler 4 in which a shaped fugitive metal 6 is deposited. Below plane XX, the support bed 8 is of granular inert material which does not wet with the molten parent metal and growing product of the oxidation reaction. In the case of aluminum parent metal at lower reaction temperatures, the support bed 8 may contain alumina particles. Inside the bed of filler 6 a base metal body 10 is placed on the support bed 8. The base metal 10 may be of any suitable shape and according to Figs. 1 and 3 has a generally rectangular flat shape having an upper side 11 opposite the lower side 13 and unlabeled. sides.

Shaped volatile metal 6 In the illustrated embodiment, the volatile metal wires have a circular cross-section. According to FIGS. 1 to 3, a washer 15 is threaded on the wire 12 at the free end and the wire 12 has a larger diameter than the cross wire 14, which in turn has a larger diameter than the wires 16a, 16b. 16c. I6d. I6e having the same diameter with each other. As can be seen from FIG. 3, the transverse wire 14 is substantially parallel to the top side 11 of the parent metal 10 as well as the wires 16a to 16e, of which only the wire 16a can be seen in FIG. The first wire 12 is not parallel to the top side 11 of the base metal 10 but is inclined with respect thereto and extends slightly upwards from the free end with the washer 15 to the point where it meets the cross wire 14. One end of each of the wires 16a to 16e contacts The individual wires may be joined to each other either by adhesive or mechanical bonding or by any metallic joint resulting, for example, by soldering or welding. Alternatively, the entire volatile metal 6 or segments thereof may be cast or otherwise formed as an integral unit. Thus, it will be appreciated that the shaped volatile metal 6 may be made in any desired shape. For example, the wire or wires may be curved and the fugitive metal may comprise or comprise shaped parts such as discs, cubes, cylinders of circular, oval or polygonal cross-section, or shaped parts such as threaded, grooved or teeth. For example, cylindrical springs made of a suitable volatile metal may be used to form a helical channel within the ceramic body. Molded parts, coiled, loop, straight or curved wires can be combined as needed to form a channel or channels of the desired shape.

The system of Figure 1 is heated to a temperature range that is high enough to melt and oxidize the parent metal 10, but is not sufficient to melt the volatile parent metal 6 or the oxidation reaction product formed from the parent metal. The gaseous oxidant passes through the bed of filler 4 in contact with the molten parent metal and oxidizes it in the temperature range indicated to produce a growing oxidation reaction product. For example, when aluminum is the parent metal, the oxidation reaction temperature may be in the range of about 800 ° C to 1450 ° C, preferably between 900 ° C and 1350 ° C, and when the oxidant is air or other oxygen-containing gas, the resulting oxidation reaction product Reaction »aluminum oxide. The molten parent metal permeates the resulting oxidation reaction product to form a ceramic body that grows to the volume shown in FIG. Even as the oxidation reaction proceeds, the volatile metal 6 is absorbed by the polycrystalline material. The reaction is allowed to proceed until the growing polycrystalline material infiltrates at least a portion of the surrounding bed of filler 4 and all or almost all of the shaped volatile metal 6. It may be advantageous for the ends of the first wire 12 and the wires 16a to 16e to extend beyond the area of ​​growth of the ceramic body resulting from the oxidation of the parent metal so that the position of the wires in the ceramic body can be seen. The volatile metal 6 is dispersed in a polycrystalline material which absorbs it, thereby forming cavities or channels in the spaces previously occupied by the volatile metal. Without wishing to be bound by theory or assumption, the shaped volatile metal appears to survive in oxidation reaction conditions long enough for the growing polycrystalline material to forcefully grow, so that on subsequent volatile metal scattering, channels that essentially negatively copy the former shape of the now dispersed shaped volatile metal 6. The dimensions of each shaped metal portion or section are at least approximately equal to those of the channels formed in the ceramic body.

Upon completion of the reaction, which preferably occurs upon complete oxidation of the parent metal so that the channel or channels formed do not become clogged with the parent metal, and upon absorption of the formed volatile metal 6 and part 7 of filler 4 by polycrystalline material, the assembly is allowed to cool. the dimensions shown in FIG. 1 are indicated by the dashed line 2 'to be separated from a possible filling 4 which may remain in the refractory vessel 2. This excess filler or part thereof may form a cohesive body, since the filler or part of the filler may itself bond at the reaction temperature.

Although the excess filler is partially sintered, it can be easily removed from the ceramic composite body by blasting, grinding, etc. The most economical way is by shot blasting using a material suitable as a filler or filler component, so that the removed filler and abrasive can then be used as a fill in the next operation. In any case, the ceramic composite body, which has one or more channels inside, is machined, ground or otherwise deformed to have the desired external shape. As shown in FIG. 4, the ceramic composite body 18 has been machined into a flat rectangular block having an upper wall 20, a front wall 22, and a rear wall 24. Inside the ceramic composite body 18 there are channels which are formed by an interconnected channel 12 of circular cross section and provided with a recess 15 'at the place where the washer 15. The channel 12' is connected to a distribution channel 14, which in turn is connected to each of the discharge channel systems. 16a, 16b. 16c. I6dI6e. One end of the discharge ducts 16a to 16e 'opens from the ceramic body 18 to the front wall 22, while the end of the duct 12 opens from the rear wall 24 of the ceramic body 18. It is evident from the illustration that the shape of the individual channels negatively follows the shape of the original shaped volatile metal 16. The channels are designated with the same reference numerals as the original wires, with only a comma. Thus, the ceramic body 18 is an article which is very suitable, for example, as a nozzle or distributor for a fluid introduced through a channel 12a passing through the discharge channels 16a 'to 16e'. All ducts are positioned and dierated precisely without the need to drill the ceramic body 18. Instead of allowing the ceramic body 18 to only grow to a rough shape and then be machined to a final shape, it can be grown to the desired shape and size by any suitable technique, for example by using a preform and filler. This eliminates the necessary grinding or other mechanical machining of the ceramic body.

If desired, the ceramic body may be allowed to grow to fully absorb the shaped volatile metal so that none of the channels formed will be open on the outer surface of the ceramic body. The ceramic body can then be opened, for example, cut off, abraded, turned, or the like so as to open at least one of the formed channels on one of the walls.

The self-supporting ceramic composite body produced by the process of the invention is useful as a liquid discharge nozzle, as a spinnerette, a metering nozzle or the like to control or facilitate the flow of a fluid such as liquid, gas, molten metal, polymer, resin and the like. The term liquid discharge nozzle refers to any type of nozzle, for example, for spraying or dispensing, extrusion nozzle, nozzles used in the melt spinning of synthetic fibers, and the like, and the spinnerette means a special nozzle commonly used in melt spinning of glass or fiber fibers. synthetic organic polymers. The volatile metal is shaped and placed in the filler according to the desired shape and dimensions having the channels that will form the flow path. The duct or ducts in the ceramic body are formed to have an inlet and an outlet opening, either by forming the volatile metal into a shape that protrudes from the area of ​​polycrystalline material formation so that the ducts are then accessible from one or more walls of the ceramic of the composite body, or by subsequently opening the composite body in which the channels are formed to access these channels. By opening a ceramic composite body is meant machining, cutting, grinding, drilling, breaking, etc. to make the channel or channels accessible from the outside. The invention makes it possible to manufacture ceramic composite articles having a complicated flow path, for example, an article having a plurality of inlet ducts that results in fewer or a single outlet duct, or a body in which the incoming flow is from a single inlet channel is distributed to several output channels. This is the case, for example, with a fuel injection nozzle in an internal combustion engine or with a spinning nozzle for spinning polymer fibers. Analogously, a complex flow path can be created by a particular arrangement of the dimensions of the drill body between the inlet and outlet channels, which, for example, allows the formation of a mixing chamber in which the individual fluids introduced into the channel are mixed by a large number of inlet channels.

Although particular embodiments of the invention have been described in detail with particular reference to aluminum as the parent metal, other parent metals, including, but not limited to, silicon, titanium, tin, zirconium, and hafnium, meet the criteria of the invention. Specific embodiments of the invention include, for example, aluminum as the parent metal and aluminum oxide or aluminum nitride as the oxidation reaction product, titanium as the parent metal and titanium nitride as the oxidation reaction product, and silicon as the parent metal and silicon carbide as the oxidation reaction product.

According to the invention, solid, liquid or gaseous oxidants or combinations thereof can be used. Typical oxidants include, but are not limited to, oxygen, nitrogen, halogen, sulfur, phosphorus, arsenic, carbon, boron, selenium, tellurium and compounds or combinations thereof, for example, silica as the oxygen source, methane, ethane, propene, acetylene, ethylene, and propylene. as carbon sources and mixtures such as air, Hg / figO and CO / CO 2, where the last two mixtures are useful in reducing environmental activity. Accordingly, the ceramic structure of the invention may comprise an oxidation reaction product consisting of one or more oxides, nitrides, carbides, borides and oxynitrides. The oxidation reaction product may be one or more compounds including aluminum oxide, aluminum nitride, silicon carbide, silicon boride, aluminum boride, titanium nitride, zirconium nitride, titanium boride, zirconium boride, zirconium carbide, silicon nitride, molybdenum eilicide, titanium carbide, carbide hafnium, hafnium boride and tin oxide.

Although specific embodiments of the invention are described in conjunction with the use of gaseous oxidants, any southern oxidant may be used. If a gaseous or vapor-phase oxidant, i.e. vapor-phase oxidant, is used to produce the ceramic body that closes the filler. the oxidant, so that when the filler bed is exposed to the oxidant, the gaseous oxidant of the filler is passed to come into contact with the molten parent metal. The term vapor-phase oxidant means normally a gaseous material or material that has been converted in vapor and produces an oxidizing atmosphere, preferably at atmospheric pressure. Oxygen or oxygen-containing mixtures including air are, for example, preferred gas phase oxidants, for example in the case of aluminum as the parent metal and alumina as the reaction product, with air generally being preferred for obvious economic reasons. Where the oxidant is said to comprise or consists of a particular gas or vapor, it is meant that the oxidant is the sole, major or at least important parent metal oxidant under conditions of oxidation reaction. Thus, although the main constituent of air is nitrogen, the oxygen contained in the air is the only oxidant for the parent metal, since oxygen is a significantly stronger oxidant than nitrogen. Thus, air falls under the term oxygen-containing oxidant and not nitrogen-containing oxidant. An example of a nitrogen-containing oxidant gas is a forming gas, which typically contains 96% nitrogen by volume and 4% hydrogen by volume.

When a solid oxidant is used, it is generally dispersed throughout the filler bed, or at least in part adjacent to the parent metal, in the form of particles mixed with the filler or as a coating on the filler particles. Any suitable solid oxidant, including elements such as boron or carbon, or reducible compounds such as silica or some borides with lower thermodynamic stability than the boride forming product of the parent metal oxidation reaction can be used. For example, when silica is used as the solid oxidant for aluminum as the parent metal, the resulting oxidation reaction product is alumina.

In some cases, the oxidation reaction may proceed so rapidly using a solid oxidant that its product tends to sinter due to the exothermic nature of the process. As a result, the uniformity of the microstructure in the ceramic body can be impaired. Such a rapid exothermic reaction can be prevented or moderated by adding relatively inert fillers having low reactivity to the mixture. An example of such a suitable inert filler material is a substance that is identical to the intended oxidation reaction product.

When a liquid oxidant is used, it impregnates the entire bed of filler or at least the portion adjacent to the molten metal. By liquid oxidant is meant a substance which is liquid under the conditions of the oxidation reaction, so that the liquid oxidant may have a solid precursor, for example a salt, which melts under the conditions of the oxidation reaction. Alternatively, the liquid oxidant may have a liquid precursor, for example a solution of the material by which the filler or part thereof is impregnated, for example by immersion, and which melts or decomposes under oxidation reaction conditions leaving a oxidation residue. Examples of liquid oxidants of this type are low melting glasses.

The filler, if used, may consist of a single material or a mixture of two or more materials, and not be dispersed into a polycrystalline material. One suitable class of fillers are those chemicals that are non-volatile at temperature and oxidation reaction conditions, are thermodynamically stable, and neither react nor excessively dissolve in the molten parent metal. In the case of aluminum as the parent metal and air or oxygen as the oxidants, these materials include oxides such as aluminum oxide AlgO4, calcium oxide CaO, sulfur dioxide COg, hafnium oxide HfOg, lanthanum oxide LagOy, lithium oxide LigO, magnesium oxide MgO, neodymium oxide NdgO ^, various praseodymium oxides, samarium SngO ^, scandium oxide SCgOp thorium oxide TbOg, uranium dioxide UOg, yttrium oxide YgO ^ ^{8 oxide}zirconium ZrOg. In addition, this class includes a large number of binary, temami and higher metal compounds, such as magnesium aluminate spinel MgO.AlgO4, which forms stable refractory compounds.

The second class of suitable fillers includes substances which are not themselves stable under the oxidation and high temperature conditions of the invention, but which, due to the slow kinetics of the degradation reactions, can be introduced as a filler into the growing ceramic body. An example of such a compound is silicon carbide. Under the conditions necessary to oxidize aluminum with oxygen or air, this material would completely oxidize if there was no protective layer of silica that forms and covers the silicon carbide particles and thus prevents further oxidation of the carbide. Thus, the silicon oxide protective layer allows the silicon carbide particles to sinter or bond together or with other filler components.

A third class of suitable filler materials are those materials, such as carbon fibers, which, due to thermodynamics or kinetics, are not expected to survive the oxidizing environment necessary to carry out the process of the invention and the action of the molten metal. by reducing the activity of the environment, for example by using a mixture of Hg / KgO or CO / COg as the oxidizing gas, or by applying a coating, for example of alumina, which it causes, the filler does not react in the oxidizing environment or in contact with the molten metal.

As described in the patent literature, dopants used in conjunction with the parent metal may in some cases favorably influence the course of the oxidation reaction, particularly in systems where the parent metal is aluminum. The function of the dopant may depend on a number of factors other than the dopant itself. These factors include, for example, a combination of impurities, when more than two impurities are used, the use of impurities deposited externally in conjunction with an impurity forming the parent metal alloy component, impurity concentration, oxidizing environment, and process conditions.

The admixture (s) used in conjunction with the parent metal may be alloying constituents of the parent metal, may be applied to at least a portion of the parent metal surface, or may be applied or introduced into the filler material, portion or filler preform, or combinations thereof of these measures. In the case where the additive (s) is / are applied to the filler, this can be done in any suitable manner.

Suitable dopants for aluminum as the parent metal, especially when air is used as the oxidant, include magnesium, zinc and silicon, either alone or in combination with each other or with other dopants. These metals or a suitable source thereof can be alloyed with an aluminum-based parent metal in an individual concentration of between 0.1 and 10% by weight based on the total weight of the doping metal. These dopant materials or a suitable source thereof, for example, magnesium oxide, zinc oxide or silica, may be used as the external dopant of the parent metal.

An alumina ceramic structure can be made from the base metal aluminum-silicon alloy using air as the oxidant when magnesium oxide is used as a surface dopant in an amount of greater than about 0.0008 g per g of parent metal and greater than about 0.003 g per cm of surface of the parent metal to which the oxide is deposited.

Other examples of dopants which are effective for aluminum as parent metal in air oxidation include sodium, germanium, tin, lead, lithium, calcium, boron, phosphorus and yttrium, which can be used singly or in combination with each other or in combination with other additives, which depends on the oxidant and process conditions. Rare earth metals such as cerium, lanthanum, praseodymium and samarium are also suitable admixtures, especially in combination with other admixtures. All dopants promote the growth of the polycrystalline oxidation reaction product in aluminum parent metal systems.

The ceramic composite structure produced by the process of the present invention is typically a dense cohesive mass wherein about 5% to about 98% of the total composite structure excluding channels is constituted by one or more filler components enclosed in a polycrystalline matrix. The polycrystalline matrix typically comprises, when the parent metal is aluminum, about 60% to 98% by weight based on the weight of the polycrystalline material, an interconnected aluminum oxide and about 1 to 40% by weight of the non-oxidized parent metal and volatile metal components.

The invention will be illustrated by the following examples.

Example 1

According to the method of the invention, a ceramic composite body with an internal helical channel was produced. The volatile metal used was a commercially available metal wire having a composition of 5 wt% Al, 22 wt% Cr, 0.5 wt% Co, the remainder Fe, a melting point of about 1510 ° C, and a diameter of 0.81 mm. A section of this wire was coiled into a helix approximately 25.4 mm long and 22.22 mm in diameter. The helix was heated in an oxygen atmosphere at 1200 ° C for 36 hours to form an oxide coating on its surface. The coiled helix was then removed and threaded onto a cylindrical aluminum alloy ingot having a nominal composition of 8 to 8.5 wt% Si, 2 to 3 wt% Zn, and 0.1 wt% to be active ingredients, 3.5 wt% Cu as well as Fe, tone, Ni, Al residue, the actual magnesium content being somewhat higher and lying in the range of 0.17 to 0.18% by weight. The ingot had a length of 25.4 mm and a diameter of 22.22 mm, so the helix threads started at one end and ended at the other end of the ingot. The helix threaded ingot was placed in a 90 mesh aluminum filler bed in a refractory vessel so that one circular ingot surface protruded slightly above the bed level. A layer of alumina particles having a mesh size of 90 mesh and substantially refractory at the process temperature against being surrounded by an oxidation reaction product consisting of alumina was deposited on the bed of the filler, and the protruding ingot surface was covered with this layer. The system was placed in an oven and heated to 1050 ° C for 5 hours. The oven temperature was then maintained at 1050 ° C in air for 48 hours, and the system was cooled over 5 hours. It was then removed from the furnace and a composite body consisting of an alumina oxidation product enclosing the alumina filler bed particles was removed from the refractory vessel. Excess non-enclosed filler material was removed from the composite surface and the ceramic composite body was cut to see the geometry of the resulting helical channel having the shape of a volatile metal helix. Giant. 5 is a photomicrograph of a cut ceramic composite at 100X magnification. As can be seen, the ee scattered the volatile metal from its original position and thereby formed a helical channel. The measured channel diameter was 0.87 mm. The slight difference between the diameter of the volatile metal wire and the diameter of the channel can be attributed to the difference between the thermal expansion of the metal wire that occurs during heating and the heat shrinkage of the composite body that occurs upon cooling.

Example 2

According to the process of the invention, a ceramic composite body was produced having four parallel internal channels. The volatile metal consisted of four sections of nickel wire with a purity of 99.9975% with a melting point of 1453 ° C, having a length of approximately 101.6 mm and a diameter of 1 mm. A rod of the same aluminum alloy as in Example 1, having a length of 114.3 nm, a width of 50.8 mm, and a height of 12.7 mm, was inserted into a bed of the same 90 mesh refractory alumina particles as in Example 1 such that an area of ​​114.3 * 50.8 mm was exposed to the atmosphere and was substantially flush with the refractory bed. A layer of 90 mesh alumina filler, about 6.35 mm deep, was laid on the free area of ​​the aluminum alloy. The four nickel wires were laid substantially parallel to the top side of the filler layer that they were substantially parallel to the top surface of the aluminum alloy bar that lay beneath them. The wires were then coated with the same filler alumina material. The assembly was then placed in an oven and heated to 1080 ° C in air for 5 hours. The oven temperature was maintained at this temperature for 48 hours and then lowered to ambient temperature over 5 hours. Then, the assembly was removed from the furnace and the resulting ceramic body consisting of an alumina oxidation reaction product in which the alumina filler was sealed was removed from the refractory vessel. The ceramic composite was then cut to see channels replacing the metal wires. Giant. 6 is a photograph of a slit composite body showing four parallel channels within the body. The measured diameter of one channel was 1.06 mm.

Example 3

The procedure described in Example 2 was repeated except that the volatile metal wires used in Example 1 were coated on the surface with a layer of chromium trioxide mixed with polyvinyl alcohol as the carrier, and then coated with a mixture of colloidal silica and 500 alumina particles. The assembly was heated in the same cycle as in Example 2 and the resulting composite was cut to show the channels formed. Giant. 7 Yeah, a photomicrograph at 50x magnification that shows the channels formed essentially by depletion of the volatile metal from its original position. Further, the photomicrograph shows an annular envelope that is substantially concentric to the channel and formed by the coating. Analysis of the coating by electron microscopy revealed that it consists practically of alumina. The channel diameter of FIG. 7 was 0.87 mm as measured.

Example 4

The procedure of Example 2 was repeated again, except that the four volatile metal wires consisted of a Ni-Cr-Al alloy having a weight composition of 16% Cr,

4.5% Al, 2% Co, 2.5% Ce, 0.5% Mo, 0.5% W, 0.05% C, 0.02% Y, 0.01% B, Ni balance, temperature melting at about 1445 ° C. The system was placed in an oven and heated to 1050 ° C for 5 hours. This temperature was maintained for 72 hours and the furnace was then cooled over 5 hours. The resulting ceramic composite was cut to see channels formed by the volatile metal volatilization from its original position and dispersed in the composite structure. Giant. 8 is a photograph of a slit ceramic composite where a light source was placed behind one of the channels formed to illuminate it and thus demonstrate the continuity of this illuminated channel.

Although only a few embodiments of the invention have been described in detail, it will be apparent to those skilled in the art that the invention encompasses a number of combinations and variations in addition to these examples.