MXPA97007064A - Synthesis of submi grain metal carbide - Google Patents

Abstract

The submicron carbide particles are formed from metal-containing precursor compositions, where the metal is tantalum, niobium, vanadium or chromium, subjecting the precursor composition to a carbonization reaction. The carbonization gas contains only hydrogen, nitrogen and carbon and the carbonization reaction is carried out at a temperature of less than 1100øC. Maintaining the carbon deposit potel of the reaction from 10% to 60%. The precursor composition can be reduced and simultaneously carburized to form the carbide. The product formed has a physical grain size of less than 1 micron, which significantly improves its ability to be ground with other compositions, as well as its use in cutting tools and the like

Description

SYNTHESIS OF SÜBMICRA GRAIN METAL CARBIDE BACKGROUND OF THE INVENTION Metal carbides are used for a variety of different applications. Some metal carbides are used as abrasive materials, while others such as vanadium carbide and chromium are inhibitors of grain growth. Other useful carbides include tantalum carbide and niobium carbide, both of which are used in cutting tools. For the most part these carbides are formed by reacting either the elemental metal or the oxide with carbon at relatively high temperatures - in any form from 1500 to 1800 ° C. The final product is a carbide which has a relatively coarse grain size, mainly due to this high reaction temperatures. This is acceptable for some applications, but it is generally preferred to have a relatively small grain size, particularly a submicron. With the cutting tools, this provides better hardness. With additives, such as grain growth inhibitors, this provides better dispersion of grain growth inhibitor and more uniform grain growth inhibition. Unfortunately, it is very difficult to form that products without using higher temperatures. The stabilization of the oxides is such that it is impossible to form the composition elemental by direct reduction with hydrogen. The compositions of tantalum, niobium, chromium and vanadium, in particular have relatively stable oxides. In turn, the carbides are relatively unstable. In this way, once an oxide of these metals is formed, it is not easy to convert them to a carbide, either directly at low temperature or through reduction to the metal followed by carbonization. BRIEF DESCRIPTION OF THE INVENTION Accordingly, an object of the present invention is to provide a method for forming chromium, vanadium, tantalum and niobium carbides. Furthermore, it is an object of the present invention to do this at a relatively low temperature, in which the grain size of the carbide formed is submicra. The objects and advantages of the present invention are achieved by reacting the chromium, vanadium, tantalum or niobium compositions at temperatures below 1100 ° C in the presence of a carbonization gas. The potential of The carbon deposit of the carbonization gas is maintained from about 10% to about 60%, whereby the carbides form directly from oxygen-containing compositions, in which the carbonization gas acts as a reducing agent and a carbon source. . The potential of carbon deposit is achieved using a gas containing Carbon, which does not contain oxygen, contains only carbon and hydrogen. The carbonization gas in turn is mixed with hydrogen and possibly, in addition, with an inert gas such as nitrogen, helium or argon. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a graph that illustrates the carbon deposition potential of various gas concentrations. DETAILED DESCRIPTION OF THE INVENTION According to the present invention, the chromium, vanadium, tantalum and niobium carbides are formed by carbonization of the precursor compositions. The precursor compositions can be any solid composition, which includes one of the target metals - chromium, vanadium, niobium or tantalum - in which the composition does not include elements other than carbon, hydrogen, nitrogen or oxygen. Primarily, these precursor compositions should not include other metals or ceramics unless desired in the final product. The organometallic compositions are particularly suitable as well as the ammonium salts of these materials, such as ammonium vanadate.

Other exemplary precursor compositions include sodium acetate hydroxide, niobium oxide and tantalum oxide. Although large particle precursor powders can be used, it is preferable that the size of the The particle size of the precursor compositions is as small as possible, preferably less than about 50 microns in size. This can be achieved by grinding or spray drying to form the desired particle size. When spray drying is employed, the precursor composition is dissolved in a solvent such as water and spray dried to form small particles. The particulate precursor composition is subjected # then to reduction / simultaneous carbonization. The reactor for this carbonization may be any of a variety of different reactors, such as a fluidized bed reactor, a rotary bed reactor or a tubular reactor with a fixed bed. The particular selection of the reactor is not critical to the present invention. The carbonization gas mixture must be a composition, which includes only an inert gas, carbon and hydrogen and preferably only carbon and hydrogen. These gases include hydrogen and lower molecular weight hydrocarbons such as methane, ethane, propane, butane, Ethylene, acetylene and the like. Inert gases include nitrogen, helium and argon. These compositions do not contain oxygen or other elements that interfere. The composition of the gas mixture is chosen to establish a lower carbon deposition potential of the 100% under the reaction conditions. The figure shows the various carbon deposition potentials for the selected gases, including ethylene and methane diluted with hydrogen and demonstrated at various reaction temperatures -110 ° C and lower. Many gas mixtures are able to deposit or gasify the carbon, for example CH4 / H2 or CO / C02 - In these two cases, the important equilibria are: CH4 (g) = 2H2 (g) + C (s) 2CO (g) ) = C02 (g) + C (s) A gas mixture composition that is at equilibrium n with solid carbon (graphite) has carbon activity unit "ac = 1". A gas mixture is said to be "stable" if its carbon activity is less than unity. Stable gas mixtures will classify the carbon and in so doing, the composition of the mixture will tend toward the composition having one unit of carbon activity. The "unstable" gas mixtures will deposit carbon and in doing so, the composition of the mixture will again tend towards the composition having carbon activity of the unit. The "attraction composition" of a gas mixture is the composition that is in equilibrium with graphite at a specified temperature and pressure, that is, the composition of the gas with the carbon activity of the unit.

The "carbon deposit potential" of an unstable gas mixture, the total carbon fraction, which is contained in the gas phase, which must be deposited in order for the mixture to achieve its attractive composition. The mixture of 1 stable gas has carbon deposit potential and zero.

Unstable gas mixtures have carbon deposit potentials between zero and one. These can also be mentioned as a percentage, where a carbon deposit potential of 1 is 100%. The carbon deposition potential of a gas mixture can be calculated. Consider a mixture of hydrogen and methane at a constant pressure, P and constant temperature T. Let P-H21 PCH41 be at pressures! initial partial hydrogen and methane. The partial pressures of hydrogen and methane at equilibrium are determined by the equilibrium reaction: j CH4 (g) = C (s) + 2 H2 (g) for which I AG = -RT in K I is the change in free energy at the given temperature, K is the equilibrium constant and R is the gas constant. If the initial gas mixture is thermodynamically unstable the partial pressures of hydrogen and methane will displace their equilibrium values, a cedida that the carbon is deposited.

The Carbon Deposit Potential (CDP) for the initial gas mixture is calculated by solving the following three equations simultaneously for the three CDPs, I? CH4 * unknown: PH2f + PCH4f = P tPH2f] 2 / tPCH4f] s exP ["? G / R] CDP-1- { T (2PH21 )] * [(PcH4f > / (2PHZf + 4pCH4f)].} These results are shown in the Figure. The potential for carbon deposition may be 20% to about 60%. At more than 50%, the free carbon is deposited, which can provide a less desirable product, that is, that has significant free carbon, ß • which must be eliminated. This, in turn, increases the porosity. At 10%, the kinetics of the reaction is too much lowers such that the time required may be unacceptable. In this way, it is preferred that it has carbon activity of about 40% to about 60%, with 40-50% being preferred. The optimum flow velocity of the diluent / carbonization gas mixture depends on the type of the reactor used, but it must be high enough to wash the gaseous reaction products from the reaction zone of the reactor. The reaction time will vary widely depending on the amount of the precursor composition used, the depth of the bed and the type of reactor. AND a fluidized bed reactor, a relatively short period of time - less than one hour - can be used, while in a fixed bed reactor the time can be from four to six hours, depending on how much it takes to achieve the infiltration of the carbonization gas within the precursor composition. The reaction temperature will be maintained from about 800 to about 1100 degrees centigrade until the desired reaction time has been achieved and the metal carbide formed. The metal carbide particles formed will have a smaller grain size than one miera (usually 50 nanopters to 1 miera), making it particularly useful for mixing with other compositions, as well as for use in cutting tools and similar. In addition, the particles themselves are very friable, which also ensures the uniform dispersion of the carbide grains during grinding or milling. The advantages of the present invention are will further appreciate in light of the following detailed examples. Example 1 One hundred fifty milligrams of chromium (III) (CH3C02) 7Cr3 (OH) 2 acetate hydroxide powder are placed in a platinum vessel for reaction with a gas mixture of hydrogen and ethylene in a controlled atmosphere thermogravimetric analyzer (TGA). The reactor is evacuated first at a pressure of 3.9 Torr and then refilled with argon. The argon atmosphere in the reactor is then displaced by a mixture of 1% ethylene in flowing hydrogen (180 cm ^ / minute). The reactor temperature is increased to 1100 ° C in 85 minutes, maintained at 1100 ° C for 55 minutes and cooled in the hydrogen flowing to 170 ° C in * 100 minutes At this point, the flow of hydrogen stops and is replaced by the flowing argon. The reactor is further cooled to room temperature in 45 minutes. The change in the weight of the sample during the reaction cycle is recorded. The X-ray diffraction analysis showed that the product of the final reaction was Cr3C2 in pure phase. He measured grain size was approximately 1 miera. Example 2 The experiment of Example 1 is repeated, but with a different temperature program. The temperature of the reactor was increased to 900 ° C in 60 minutes, maintained at 900 ° C during 125 minutes and cooled to 650 ° C in 30 minutes, then at room temperature for an additional 60 minutes. The atmosphere of 1% ethylene in hydrogen is maintained throughout the experiment, except that the sample is cooled in the flowing hydrogen. The change in the weight of the sample during the reaction cycle is recorded. The analysis of X-ray diffraction showed that the final reaction product was pure phase Cr3Cr2. Example 3 The experiment of Example 1 was repeated, but with a different temperature program. The temperature of the reactor was increased to 800 ° C in 48 minutes and maintained at 800 ° C for 186 minutes. Since the reaction does not proceed to its final stage at an appreciable speed, the temperature was increased to 850 ° C in 3 minutes. The temperature of the reactor is kept at 850 ° C for 90 minutes and cooled to room temperature in an additional 30 minutes. The atmosphere of 1% ethylene in hydrogen is maintained throughout the reaction. X-ray diffraction analysis showed that the final reaction product was Cr3Cr2 in pure phase. Example 4 149.3 milligrams of chromium (III) hydroxide powder, (CH3C0) 7Cr3 (OH) 2 is placed in a platinum vessel for reaction with a mixture of hydrogen gas and ethylene in a thermogravimetric analyzer of controlled atmosphere (TGA). The reactor is evacuated first to a pressure of 3.7 Torr and then refilled with argon. The argon atmosphere in the reactor is then displaced by a flow mixture (180 cm3 / minute) of 5.5% ethylene in hydrogen. The temperature of the reactor is increased to 700 ° C in 60 minutes, it is maintained at 700 ° C for 8 hours 40 minutes.

The ethylene flow stops and the reactor is heated to 750 ° C. At this point, the hydrogen / ethylene flow is stopped and replaced by the argon flow. The reactor is further cooled to room temperature in 45 minutes. The change in the weight of the sample during the reaction cycle is recorded. The X-ray diffraction analysis showed that the product of the final reaction was Cr3C2 almost pure phase, but with some Cr7C3. The measured grain size was approximately 0.5 microns. Example 5 Synthesis of VC from NBJ4V03 Using H2-C2BJ4 at 1100 ° C 587.8 milligrams of ammonium vanadate powder, NH4V03, are placed in a platinum vessel for reaction with a gaseous mixture of hydrogen and ethylene in a thermogravimetric analyzer. of controlled atmosphere (TGA). First the reactor is evacuated to a pressure of 3.9 Torr and then refilled with argon. The argon atmosphere in the reactor is displaced by a flow mixture (180 cm3 / minute) of 1% ethylene in hydrogen. The The reactor temperature rises to 1100 ° C in 85 minutes, is maintained at 1100 ° C for 120 minutes and cooled to less than 400 ° C in 10 minutes. At this point, the hydrogen / ethylene flow is stopped and replaced by the flow of Ar: H2 1: 1. The reactor is also cooled to the temperature atmosphere during the night. The change in the weight of the sample during the reaction cycle is recorded. X-ray diffraction analysis showed that the product of the final reaction was pure phase VC. The measured grain size was approximately 250 nanometers. Example 6 The procedure of Example 5 was followed, except that the reaction temperature was lowered to 800 ° C. The pure phase VC was produced with a grain size of In this manner, using the present invention, submicron particles can be made which include tantalum carbide, niobium carbide, chromium carbide and vanadium carbide.These particular compositions, in turn, are useful in virtually every application where eetos carbides have been previously used. Due to their particle sizes, they can be easily mixed and are more effective. In addition, when used with cutting tools, they provide a cutting tool with euperioree properties. This has been a description of the present invention, together with the currently known preferred method of practicing the present invention. However, the invention by itself should only be defined by the appended claims, in which it claims:

Claims (18)

1. CLAIMS 1. A method for forming a metal carbide of a first metal selected from the group consisting of tantalum, niobium, vanadium and chromium, characterized in that it comprises reacting a precursor composition formed from the metal, the carbonization composition with a gae mixture containing carbon, the mixture of gae containing rafe carbon contains diluent and a carbonization gae, the carbonization gae is composed of atoms eeleccionare of group 10 that you connected of carbon, hydrogen and nitrogen and in which the potential of carbon deposit is maintained from about 10% to about 60% and wherein a reaction temperature is set from about 800 ° to about 1100 ° C.
2. 2. The method according to claim m, characterized in that the first metal is tantalum.
3. 3. The method according to claim 1, characterized in that the first metal is niobium.
4. 4. The method according to claim 20 1, characterized in that the first metal is vanadium.
5. 5. The method according to claim 1, characterized in that the first metal is chromium.
6. 6. The method according to claim 1, characterized in that the carbonization gas ee selects from the group consisting of methane, ethane, propane, butane, ethylene and acetylene.
7. 7. The method according to claim 1, characterized in that the precursor composition is an organometallic compound.
8. 8. The method according to claim 1, characterized in that the precursor composition is an ammonium salt.
9. The method according to claim 1, characterized in that the carbon deposition potential is maintained from about 40% to about 50%.
10. 10. A product produced according to the method according to claim 1, characterized in that the metal carbide has a grain size of less than 1 miera.
11. 11. The product produced by the method according to claim 2, characterized in that the metal carbide has a grain size of less than 1 miera.
12. 12. The product produced by the method according to claim 3, characterized in that the metal carbide has a grain size of less than 1 miera.
13. 13. The product produced by the method according to claim 4, characterized in that the Metal carbide has a grain size of less than 1 miera.
14. 14. The product produced by the method according to claim 5, characterized in that the metal carbide has a grain size of less than 1 miera.
15. 15. The product produced by the method according to claim 6, characterized in that the metal carbide has a grain size of less than 1 miera.
16. 16. The product produced by the method according to claim 7, characterized in that the metal carbide has a grain size of less than 1 millimeter.
17. 17. The product • produced by the method according to claim 8, characterized in that the metal carbide has a grain size of less than 1 miera.
18. 18. The product produced by the method according to claim 9, characterized in that the metal carbide has a grain size of less than 1 miera.