Patent Application
20100055017
The present invention relates to the production of ultrafine metal carbide particles, and more particularly relates to a method of producing such particles which generates hydrogen for recirculation into the ultrafine particle-forming process and for other uses.
Boron carbide particles having particle sizes of greater than 0.2 micron have been produced by solid phase synthesis using B2O3 and carbon as starting reactant materials and subsequent milling. Such particles may be sintered to form various products such as armor panels and abrasion resistant nozzles. Sintering aids may be added to such boron carbide particles by milling in order to obtain a mixture that is homogeneous on a macro scale. However, these mixtures are not uniform on a microscale, and such non-uniformities may adversely affect sintering of the particles and cause defects in the sintered bodies that degrade mechanical properties.
In certain respects, the present invention is directed to providing a method for producing ultrafine metal carbide particles and hydrogen comprising: introducing a metal-containing precursor and a carbon-containing precursor into a thermal reaction chamber; heating the precursors in the thermal reaction chamber to form the ultrafine metal carbide particles from the precursors and to form carbon monoxide and hydrogen; collecting the ultrafine metal carbide particles; converting at least a portion of the carbon monoxide to carbon dioxide and generating additional hydrogen; and recovering at least a portion of the hydrogen.
In other respects, the present invention is directed to providing a system for producing ultrafine metal carbide particles and hydrogen comprising: a source of metal-containing precursor; a source of carbon-containing precursor; means for introducing the metal-containing precursor and carbon-containing precursor into a thermal reaction chamber to form the ultrafine metal carbide particles from the precursors and to form carbon monoxide and hydrogen; and means for converting at least a portion of the carbon monoxide to carbon dioxide and generating additional hydrogen.
For purposes of the following detailed description, it is to be understood that the invention may assume various alternative variations and step sequences, except where expressly specified to the contrary. Moreover, other than in any operating examples, or where otherwise indicated, all numbers expressing, for example, quantities of ingredients used in the specification and claims are to be understood as being modified in all instances by the term “about”. Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the desired properties to be obtained by the present invention. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.
Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard variation found in their respective testing measurements.
Also, it should be understood that any numerical range recited herein is intended to include all sub-ranges subsumed therein. For example, a range of “1 to 10” is intended to include all sub-ranges between (and including) the recited minimum value of 1 and the recited maximum value of 10, that is, having a minimum value equal to or greater than 1 and a maximum value of equal to or less than 10.
In this application, the use of the singular includes the plural and plural encompasses singular, unless specifically stated otherwise. In addition, in this application, the use of “or” means “and/or” unless specifically stated otherwise, even though “and/or” may be explicitly used in certain instances.
Certain embodiments of the present invention are directed to methods for making ultrafine metal carbide particles and generating hydrogen for recirculation into the particle-forming process or for other uses. Examples of ultrafine metal carbides that may be produced include boron carbides such as B4C, B13C2, B8C, B10C, B25C. Other ultrafine doped metal carbides that may be produced in accordance with the present invention include tungsten carbide, titanium carbide, silicon carbide, aluminum carbide, iron carbide, zirconium carbide, magnesium aluminum carbide, hafnium carbide and the like.
As used herein, the term “ultrafine metal carbide particles” refers to metal carbide particles having a B.E.T. specific surface area of at least 5 square meters per gram, such as 20 to 200 square meters per gram, or, in some cases, 30 to 100 square meters per gram. As used herein, the term “B.E.T. specific surface area” refers to a specific surface area determined by nitrogen adsorption according to the ASTMD 3663-78 standard based on the Brunauer-Emmett-Teller method described in the periodical “The Journal of the American Chemical Society”, 60, 309 (1938).
In certain embodiments, the ultrafine metal carbide particles made in accordance with the present invention have a calculated equivalent spherical diameter of no more than 200 nanometers, such as no more than 100 nanometers, or, in certain embodiments, 5 to 50 nanometers. As will be understood by those skilled in the art, a calculated equivalent spherical diameter can be determined from the B.E.T. specific surface area according to the following equation:
Diameter (nanometers)=6000/[BET(m2/g)*ρ(grams/cm3)]
In certain embodiments, the ultrafine metal carbide particles have an average particle size of no more than 100 nanometers, in some cases, no more than 50 nanometers or, in yet other cases, no more than 30 or 40 nanometers. As used herein, the term “average particle size” refers to a particle size as determined by visually examining a micrograph of a transmission electron microscopy (“TEM”) image, measuring the diameter of the particles in the image, and calculating the average particle size of the measured particles based on magnification of the TEM image. One of ordinary skill in the art will understand how to prepare such a TEM image and determine the average particle size based on the magnification. The size of a particle refers to the smallest diameter sphere that will completely enclose the individual particle.
In certain embodiments, the boron-containing precursor may be provided in solid particulate form. For example, ultrafine boron carbide particles may be produced from B2O3 precursor powders. In certain embodiments, the carbon source may be carbon powder or a gas such as methane or natural gas. For example, boron-containing compounds such as B2O3 or borax particles may be suspended or dissolved in a carbon-containing gas such as methane or natural gas, or in an organic liquid such as methanol, glycerol, ethylene glycol or dimethyl carbonate.
In one embodiment, the metal-containing and carbon-containing precursors may be provided in liquid form. The term “liquid precursor” means a precursor material that is liquid at room temperature. In accordance with certain embodiments in which boron carbide powders are produced, suitable liquid boron-containing precursors include borate esters and other compounds containing boron-oxygen bonds. For example, the liquid boron-containing precursor may comprise trimethylboroxine, trimethylborate and/or triethylborate. The carbon-containing precursor may be in liquid form and may comprise aliphatic carbon atoms and/or aromatic carbon atoms. For example, the liquid carbon-containing precursor may comprise acetone, iso-octane and/or toluene. In certain embodiments, the liquid carbon-containing precursor may comprise an organic liquid with a relatively high C:H atomic ratio, e.g., greater than 1:3 or greater than 1:2. Furthermore, such liquid hydrocarbon precursors may also have a relatively high C:O atomic ratio, e.g., greater than 2:1 or greater than 3:1.
When B2O3 and C powders are used as the precursors in the presence of a H2 carrier gas, the following formula applies:
2B2O3(s)+7C(s)→B4C(s)+6CO(g).
When methane is used as the carbon-containing precursor, the following formula applies:
2B2O3(s)+7CH4(g)→B4C(s)+6CO(g)+14H2(g).
In accordance with certain embodiments, the ratio of boron-containing precursor to carbon-containing precursor is controlled in order to control the composition of the resultant boron carbide and/or in order to control the formation of excess boron or excess carbon in the ultrafine boron carbide particles. For example, if an excess amount of boron-containing precursor is used, excess boron may form on or in the ultrafine boron carbide particles, which may react with oxygen or air to form oxide compounds. As a further example, an excess amount of carbon-containing precursor in the starting feed material may cause the formation of graphite on or in the resultant boron carbide particles.
As shown in
Next, in accordance with certain embodiments of the present invention, the precursors and carrier gas are heated by a plasma system, e.g., as the entrained precursors flow into a plasma chamber, yielding a gaseous stream of the precursors and/or their vaporized or thermal decomposition products and/or their reaction products. In certain embodiments, the precursors are heated to a temperature ranging from 1,500° to 20,000° C., such as 1,700° to 8,000° C. Any conventionally known type of plasma gas may be used. In one embodiment, the plasma gas comprises hydrogen which is generated by the process and recirculated into the system.
In certain methods of the present invention, after the gaseous stream is produced, it is contacted with one or more quench streams that are injected into the plasma chamber through at least one quench stream injection port. For example, the quench streams are injected at flow rates and injection angles that result in impingement of the quench streams with each other within the gaseous stream. The material used in the quench streams is not limited, so long as it adequately cools the gaseous stream to facilitate the formation or control the particle size of the ultrafine doped metal carbide particles. Materials suitable for use in the quench streams include, but are not limited to, inert gases such as argon, helium, nitrogen, hydrogen gas, ammonia, mono, di and polybasic alcohols, hydrocarbons, amines and/or carboxylic acids. In one embodiment, the quench gas comprises hydrogen which is generated during the process and recirculated into the system.
In certain embodiments, the particular flow rates and injection angles of the various quench streams may vary, for example, they may impinge with each other within the gaseous stream to result in the rapid cooling of the gaseous stream. For example, the quench streams may primarily cool the gaseous stream through dilution, rather than adiabatic expansion, thereby causing a rapid quenching of the gaseous stream, before, during and/or after the formation of the ultrafine metal carbide particles prior to passing the particles into and through a converging member, such as a converging-diverging nozzle, as described below.
In certain embodiments of the invention, after contacting the gaseous product stream with the quench streams to cause production of ultrafine metal carbide particles, the ultrafine particles may be passed through a converging member, wherein the plasma system is designed to minimize the fouling thereof. In certain embodiments, the converging member also comprises a diverging section, e.g., a converging-diverging (De Laval) nozzle. In these embodiments, while the converging-diverging nozzle may act to cool the product stream to some degree, the quench streams perform much of the cooling so that a substantial amount of the ultrafine metal carbide particles are formed upstream of the nozzle. In these embodiments, the converging-diverging nozzle may primarily act as a choke position that permits operation of the reactor at higher pressures, thereby increasing the residence time of the materials therein. A sheath gas may be introduced at or upstream from the converging-diverging nozzle. In one embodiment, the sheath gas comprises hydrogen which is generated by the process and recirculated into the system.
As is seen in
As also seen in
6CO(g)+6H2O(g)→6CO2(g)+6H2(g).
This treatment of CO and H2 gases may take place at a facility different from the facility at which the powder product is produced. In some cases, however, the facility at which the powder product is produced is integral with facility at which the CO and H2 gases are treated, such as may be the case with, for example, an existing H2 gas production facility or a facility that utilizes CO2, such as a silica production facility. In certain embodiments, therefore, the methods and systems are desirably employed at an existing hydrogen gas production facility.
As seen in
In the embodiment depicted by
A plasma is a high temperature luminous gas which is at least partially (1 to 100%) ionized. A plasma is made up of gas atoms, gas ions, and electrons. A thermal plasma can be created by passing a gas through an electric arc. The electric arc will rapidly heat the gas by resistive and radiative heating to very high temperatures within microseconds of passing through the arc. The plasma is often luminous at temperatures above 9,000 K.
A plasma can be produced with any of a variety of gases. This can give excellent control over any chemical reactions taking place in the plasma as the gas may be inert, such as argon, helium or neon, reductive, such as hydrogen, methane, ammonia or carbon monoxide, or oxidative, such as oxygen, nitrogen or carbon dioxide.
As the gaseous product stream exits the plasma 29, it proceeds towards the outlet of the plasma chamber 20. An additional reactant, as described earlier, can optionally be injected into the reaction chamber prior to the injection of the quench streams. A supply inlet for the additional reactant is shown in
As is seen in
In certain methods of the present invention, contacting the gaseous stream with the quench streams may result in the formation and/or control of the particle size of the ultrafine particles, which are then passed into and through a converging member. As used herein, the term “converging member” refers to a device that restricts passage of a flow therethrough, thereby controlling the residence time of the flow in the plasma chamber due to pressure differential upstream and downstream of the converging member.
In certain embodiments, the converging member comprises a converging-diverging (De Laval) nozzle, such as that depicted in
As the confined stream of flow enters the diverging or downstream portion of the nozzle 22, it is subjected to an ultra fast decrease in pressure as a result of a gradual increase in volume along the conical walls of the nozzle exit. By proper selection of nozzle dimensions, the plasma chamber 20 can be operated at atmospheric pressure, or slightly less than atmospheric pressure, or, in some cases, at a pressurized condition, to achieve the desired residence time, while the chamber 26 downstream of the nozzle 22 is maintained at a vacuum pressure by operation of a vacuum producing device, such as a vacuum pump 60. Following passage through nozzle 22, the ultrafine metal carbide particles may then enter a cool down chamber 26.
As is apparent from
As shown in
In certain embodiments, the residence times for materials within the plasma chamber 20 are on the order of milliseconds. When the metal-containing and carbon-containing precursors are provided in liquid form, they may be injected under pressure (such as from 1 to 300 psi) through a small orifice to achieve sufficient velocity to penetrate and mix with the plasma. In addition, in many cases the injected liquid stream is injected normal (90° angle) to the flow of the plasma gases. In some cases, positive or negative deviations from the 90° angle by as much as 30° may be desired.
In the embodiment depicted by
In
As is seen in
In certain embodiments of the present invention, such as is depicted in
By proper selection of the converging member dimensions, the plasma chamber 120 can be operated at atmospheric pressure, or slightly less than atmospheric pressure, or, in some cases, at a pressurized condition, to achieve the desired residence time, while the chamber 126 downstream of the converging member 122 is maintained at a vacuum pressure by operation of a vacuum producing device, such as a vacuum pump 60. Following production of the ultrafine particles, they may then enter a cool down chamber 26. The system may be operated under vacuum or under pressure. For example, a vacuum pump may be used downstream from the particle separator to provide vacuum to the system. Alternatively, the system can be purged with gas to increase the pressure.
As is apparent from
As shown in
The precursors may be injected under pressure (such as greater than 1 to 100 atmospheres) through a small orifice to achieve sufficient velocity to penetrate and mix with the plasma. In addition, in many cases the injected stream of precursors is injected normal (90° angle) to the flow of the plasma gases. In some cases, positive or negative deviations from the 90° angle by as much as 30° may be desired.
The high temperature of the plasma may rapidly decompose and/or vaporize the precursors. There can be a substantial difference in temperature gradients and gaseous flow patterns along the length of the plasma chamber. It is believed that, at the plasma arc inlet, flow is turbulent and there is a high temperature gradient from temperatures of about 20,000 K at the axis of the chamber to about 375 K at the chamber walls. At the nozzle throat, it is believed, the flow is laminar and there is a very low temperature gradient across its restricted open area.
The plasma chamber is often constructed of water cooled stainless steel, nickel, titanium, copper, aluminum, or other suitable materials. The plasma chamber can also be constructed of ceramic materials to withstand a vigorous chemical and thermal environment.
The plasma chamber walls may be internally heated by a combination of radiation, convection and conduction. In certain embodiments, cooling of the plasma chamber walls prevents unwanted melting and/or corrosion at their surfaces. The system used to control such cooling should maintain the walls at as high a temperature as can be permitted by the selected wall material, which often is inert to the materials within the plasma chamber at the expected wall temperatures. This is true also with regard to the nozzle walls, which may be subjected to heat by convection and conduction.
The length of the plasma chamber is often determined experimentally by first using an elongated tube within which the user can locate the target threshold temperature. The plasma chamber can then be designed long enough so that the materials have sufficient residence time at the high temperature to reach an equilibrium state and complete the formation of the desired end products.
The inside diameter of the plasma chamber may be determined by the fluid properties of the plasma and moving gaseous stream. It should be sufficiently great to permit necessary gaseous flow, but not so large that recirculating eddys or stagnant zones are formed along the walls of the chamber. Such detrimental flow patterns can cool the gases prematurely and precipitate unwanted products. In many cases, the inside diameter of the plasma chamber is more than 100% of the plasma diameter at the inlet end of the plasma chamber.
In certain embodiments, the converging section of the nozzle has a high aspect ratio change in diameter that maintains smooth transitions to a first steep angle (such as >45°) and then to lesser angles (such as <45°) leading into the nozzle throat. The purpose of the nozzle throat is often to compress the gases and achieve sonic velocities in the flow. The velocities achieved in the nozzle throat and in the downstream diverging section of the nozzle are controlled by the pressure differential between the plasma chamber and the section downstream of the diverging section of the nozzle. Negative pressure can be applied downstream or positive pressure applied upstream for this purpose. A converging-diverging nozzle of the type suitable for use in the present invention is described in U.S. Pat. No. RE37,853 at col. 9, line 65 to col. 11, line 32, the cited portion of which being incorporated by reference herein.
It will be readily appreciated by those skilled in the art that modifications may be made to the invention without departing from the concepts disclosed in the foregoing description. Such modifications are to be considered as included within the following claims unless the claims, by their language, expressly state otherwise. Accordingly, the particular embodiments described in detail herein are illustrative only and are not limiting to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof.
This invention was made with United States government support under Contract Number W911NF-05-9-0001 awarded by DARPA. The United States government has certain rights in this invention.
