DE1227374B - Boron containing materials and processes for their manufacture - Google Patents

Description

Boron-Containing Materials and Processes for Their Manufacture The invention relates to boron-containing materials and processes for their production.

Boron and boron compounds are brittle materials with little mechanical Strength that has little resistance to thermal stress exhibit. Also the compounds or alloys of boron with many other common ones Metals are very brittle. The use of boron or boron compounds, for example as controls for nuclear reactors (where the control effect is known to be based on the Neutron capture is based on boron), is therefore only possible to a limited extent. target the invention is therefore a method for producing boron-containing materials with improved mechanical properties.

The boron-containing material according to the invention has a metallic one Binder or a (macroscopic) metal matrix in which the particles are made Boron or boron-containing material are included, which are provided with a coating. This coating does not undergo any chemical reaction with the particles themselves, nor does it chemical reaction with the binder and prevents a chemical reaction between the particles and the binder.

Preferred boron compounds are boron carbide and zicron diboride. Since boron carbide is very hard, materials in which boron carbide is incorporated can be underneath Good to use for cutting and drilling tools or for reactor control rods. The coating of the particles can contain compounds that are derived from the carbides, the nitrides and carbonitrides of the elements from the IV., V or VI. Group of the Periodic Table are selected. Particularly suitable substances are the heat-resistant carbides of silicon, titanium, zircon, tantalum, vanadium, chromium, Molybdenum and tungsten as well as silicon nitride. Silicon carbide is used as a coating for boron carbide particularly suitable because of the thermal expansion coefficients of these two Substances are very similar. Titanium and tungsten carbide, on the other hand, are for chemical reasons suitable.

The coating can consist of two layers, one of which is Has properties that are particularly favorable in relation to the material of the particle are, while the properties of the second layer in terms of the metal of the Binder are selected. For example, silicon carbide reacts with something Stole. It can therefore be beneficial to coat boron carbide particles first to be coated with silicon carbide and then to apply a layer of titanium carbide, which reacts with steel to a lesser extent, but in terms of its thermal expansion coefficient does not match boron carbide quite as well.

When it is said here that the coating neither with the particle nor reacts with the material of the binder, this is not intended to mean that even under the most extreme bonds no detectable chemical reactions whatsoever occur. Rather, it is only intended to say that the chemical reactions slower under normal conditions of use than between the uncoated Particles and the binder run off and also only to such a small extent as the protective layer created by the coating is retained everywhere and at all times.

Examples of metals that can be used as binders are Iron, cobalt, nickel, chromium and zircon and alloys of several or all these metals. The binder can also contain tungsten, titanium or tantalum carbide. That is particularly beneficial if the material is used for drilling or cutting tools to be used: -In the manufacture of the materials according to the invention can the small particles after a fluidized bed process with a. Cover provided will. In this fluidized bed process, a gas stream that is at high temperatures the coating produced, introduced into a heated vortex of dust that emerges from them consists of small particles. This gas flow can consist of just a single gaseous There is a compound that decomposes in the heat. But the gas flow can also contain several gases that react with each other and thereby the coating material form. The coated particles can then by means of a conventional powder metallurgical process Procedure to be incorporated into the binder: The coating should be from the material of the binder can be wetted, so that during sintering above the melting point of the binder - a mechanical, strong and dense material is obtained. As an exemplary embodiment is now the production of boron-containing materials according to the invention for two 'various purposes are described in detail.

Boron is relatively cheap and comes in. Compared to others Materials that have a high absorption cross-section compared to thermal neutrons exhibit relatively frequently. Hence, boron is very common for the scheme and control of neutron absorption in nuclear reactors. Neutron capture on boron leads to the cleavage of boron -in LiT- and He4: these two in the process of cleavage The resulting isotopes are characterized by a low neutron absorption cross-section the end. Boron can therefore be used in a nuclear reactor as a fissile or burnable »reactor poison« be used. If boron or solid, temperature-resistant boron compounds are used as Control rods are used in a reactor, they are usually in a sleeve housed the necessary protection against mechanical and thermal Tension.

Boron alloys have also been used in reactors: boron however, and boron compounds form with most metals of interest, such as for example with iron, nickel, zirconium, titanium and chromium, very brittle alloys or connections. The boron content of such alloys or compounds can therefore only be small and, for example, less than 4 percent by weight.

A similar difficulty arises when one; Particles from Boron or a boron compound using a powder metallurgical process in incorporates a metallic binder, since at the sintering and manufacturing temperatures Boron and the material of the binder can react with one another. These ; difficulty is now substantially reduced if one uses the invention and the boron-containing ones Small particles coated with one or more of the carbides already listed above have been.

Attempts have already been made to extract nuclear fuel particles from U02 with a Layer made of niobium carbide NbC by using the fluidized bed process to about = draw. The aim of these attempts was mainly to provide a coating for the nuclear fuel particles to create the fission products in the nuclear fuel particles hold back and also chemical reactions between the fuel particles and other elements of the fuel system of a nuclear reactor = should prevent.

However, these attempts have not led to any useful results.

As an example, it will now be described in detail how the boron-containing Particles can be coated with silicon carbide. A fluidized bed containing boron Particles that should, if possible (but not necessarily) have a round shape and whose diameter is in the range between 50 and 500 microns, is at one Temperature between 1000 and 1700 ° C swirled through by a hydrogen gas stream and held in abeyance. The preferred temperature is -1500 ° C: One for it a suitable boron-containing compound is boron carbide (B4C). It then becomes a gaseous one Compound, such as trichloromethylsilane CH.SiCL3, introduced. This connection decomposes at the prevailing temperatures in the presence of hydrogen and forms a dense deposit of silicon carbide.

Other silicon compounds can also be used. So you can, for example a silicon halide such as silicon tetrachloride in combination with a hydrocarbon such as using methane.

When the silicon carbide is deposited on the particles has, the particles are built into a binder. To do this, you use yourself a common powder metallurgical process. The resulting material is through Pressing, forging; Rolling or drawing processed further.

Silicon carbide turns into at temperatures below 2000 ° C its cubic. Modification, i.e. in its ß-modification. Resilience this modification to radioactive radiation is known to be very high: When choosing the material for the particles, it is advantageous to use one that contains boron Choose a material whose coefficient of thermal expansion is as good as possible corresponds to the thermal expansion coefficient of silicon carbide, in order to avoid temperature stresses in the coating that could lead to cracking of the coating could lead.

The coefficient of thermal expansion of silicon carbide is in Temperature range between 0 and 1000 ° C at 4.4 - 10-s / ° C. The expansion coefficient of boron carbide is in the same temperature range 4.5 - 10-s / ° C, of zirconium diboride 5.5 10-6 / ° C and for boron is between 1-, 1 and 8.3 - 10-6 / ° C.

The expansion coefficient of boron carbide therefore comes from the expansion coefficient of silicon carbide extraordinarily - close. Instead of boron carbide, one can also use Use zirconium diboride as the thermal expansion coefficient of zirconium boride still sufficiently good with the thermal expansion coefficient of silicon carbide conforms to sufficient uniformity of thermal expansion of these two substances. If now- the silicon carbide by the thermal Decomposition of trichloromethylsilane is produced in this reaction at the same time hydrochloric acid, which can attack the zirconium diboride. This difficulty but you can get around if you first make a coating of carbon. That Material produced by the process described above; can be extraordinary use well as regulation and control material in a nuclear reactor. It doesn't matter whether this material is used for the control rods or whether it is uniform in the reactor occurs distributed so as to act as a burnable reactor poison.

If you want to use such materials in nuclear reactors, it is it is often advantageous to use stainless steel as the material for the binder. In such a case, the temperature required to manufacture the material may be after powder metallurgical process is required to be high so that the steel and the silicon carbide can react with each other to a certain extent. This difficulty can be avoided by applying a second layer of a different carbide, which has a thermal expansion coefficient similar to that of silicon carbide. Titanium carbide is an example of this. The process by which this second coating is essentially the same as that already discussed in connection with the production of silicon carbide coatings has been described.

In some cases it is possible to use the silicon carbide coating for reactor applications and to replace this coating with a coating made of titanium carbide.

Silicon carbide shows good corrosion resistance to air, Steam and carbon dioxide up to 1500 ° C, so that even if in one Fuel rod sleeve fails and the coolant penetrates the binder, the coolant is not poisoned by material containing receipts.

If you provide the particles with two coatings, you can take advantage of these advantages of silicon carbide even at high manufacturing temperatures required are.

A second area of application of the invention is in cutting and Drilling tools. In terms of hardness, bon carbide is only found in diamond and cubic Exceeded boron nitride. Diamonds are used for numerous purposes, such as the cutting and drilling of hard metals, used with success, however, is yours Use is limited in terms of costs, while cubic boron nitride can be up to cannot be produced commercially today.

Bon carbide could be used in cutting or drilling tools, if it could be incorporated into a binder as hard carbide. A disadvantage however, this material resides in that it reacts with most metals and Forms borides that are similar to the original carbide in terms of mechanical strength and are inferior in hardness. This reaction also needs the material of the binder on which one is dependent for the toughness because of the toughness, copper and aluminum react Although not with bon carbide, they cannot be used as a binder for materials used for drilling and machining of hard materials because they are too soft for these purposes.

Boron carbide particles of various sizes, ranging in diameter from 10 to 3000 microns, can be coated with titanium carbide using a fluidized bed or fluidized bed process. According to this process, the boron carbide particles are whirled up with titanium tetra-chloride and toluene vapors in hydrogen as a carrier gas at temperatures above 900 ° C: For example, round boron carbide particles with a diameter of about 100 microns can be placed in a tube of about 2.5 Enter cm diameter and the particles by introducing hydrogen at a rate of 1 1 / min. stir it up and keep it in suspension. Before the main hydrogen stream enters the tube, two further hydrogen streams are fed to it, each of which has a flow rate of 100 cc / min. having. Before these two additional hydrogen streams are now combined with the main hydrogen stream, one is passed through titanium tetrachloride at about 231 C and the other through toluene at about 0 ° C. The fluidized and suspended particles are kept at a temperature of around 1200 ° C.

The coated particles are then mixed with cobalt, cold pressed and sintered at a temperature between 1300 and 1800 ° C.

Claims (1)

Claims: 1. Boron-containing material that can be used n -z e i c h n e t that it has a metallic binder in the particle of boron or a bon-containing material are included, which are provided with a coating that chemically reacts neither with the binding agent nor with the particles and prevents reaction between the particles and the binder. z. Receipt-containing Material according to claim 1, characterized in that the coating has at least one Has compound that leads to the carbides, the nitrides or the carbonitrides of the Elements from IV., V or VI. Belongs to group of the periodic table. 3. Receipt-containing material according to claim 2, characterized in that the connection is a refractory carbide. 4. Receipt-containing material according to claim 3, characterized characterized in that the coating consists of a layer of silicon carbide, titanium carbide or tungsten carbide. 5. Material containing receipts according to one or more of the Claims 1 to 4, characterized in that the particles are boron carbide particles are. 6. Receipt-containing material according to claim 5, characterized in that the particles Boron carbide particles are coated with titanium carbide. 7. Material containing receipts according to one or more of claims 1 to 6, characterized in that the The binder is made of stainless steel or cobalt. B. Material containing receipts according to claim 1, characterized in that it is a stainless steel binder having, are contained in the boron carbide particles with a layer of Silicon carbide and coated with another layer of titanium carbide. 9. Process for the production of a receipt-containing material according to one or more of the Claims 1 to 8, characterized in that the particles are characterized with a Cover provided be that in the heated, whirled up suspended particles a gas stream is introduced, which in the Temperature at which the particles are located, which creates the material of the coating, and then the coated particles sintered into the binder will. Publications considered: Report BMI 1440 by J. B 1 o c h e r u. a.