Patent Application
20070277648
Although authorities have yet to agree on precise definitions of “nanopowders” and ultrafine powders, for the purposes of this specification, such powders are composed of metal particles having a typical mean particle diameter in the range of about 1 to 100 nm.
As discovered by Mond and Langer during the latter part of the 19th century, nickel freely combines with and disassociates from carbon monoxide. By decomposing nickel carbonyl (Ni(CO)4), an exquisitely pure form of nickel can be produced. The main reaction is:
Ni(CO)→Ni+4CO
with a heat of reaction requirement of 160.4 kJ/mole.
Due to the high amount of energy available in the resultant plasma (64 kW) and the low energy required to decompose nickel carbonyl into nickel and carbon monoxide, the induction plasma torch 10 provides an excellent platform to generate nickel and other metal nanopowders.
Since the decomposition temperature of gaseous nickel carbonyl is about 200° C. as compared to the melting temperature of nickel powder (1453° C.) the present carbonyl based process requires significantly less power than conventional solid metallic feed processes. This means that for a given plasma power, increased production levels can be realized when using nickel carbonyl as the feed compound when compared to nickel powder as feed. By combining high plasma temperatures and low decomposition temperatures, high heating and quench rates follow. This results in fast nucleation and the production of small particles having improved spherical morphology and crystalinity.
In a preferred embodiment of the present invention, metal carbonyl gas along with a carrier or diluting gas such as helium, argon, nitrogen, hydrogen, carbon monoxide, etc. either solely or in combination are axially introduced from supply 12 into central conduit 14 of the torch 10. A plasma gas such as helium, argon, nitrogen, hydrogen, carbon monoxide, etc. either solely or in combination from plasma gas source 16 is applied to the torch 10 via conduit 18 for the purpose of magnetic coupling of gas to form plasma. A sheath gas such as helium, argon, nitrogen, hydrogen, carbon monoxide, etc. either solely or in combination is supplied to the torch 10 via conduit 22 from the sheath gas supply 20. The sheath gas insulates the carbonyl from the hot inner wall of the torch 10 and, if desired, influences the mixing patterns of the torch 10.
Cooling water is introduced to circulate around an RF induction coil 24 through input port 26 whereupon it exits at cooling water output port 28.
Upon energizing the torch 10, the metal carbonyl gas is introduced into a chamber 32 via the central conduit 14.
The metal carbonyl is subjected to an extremely rapid decomposition and quench below the terminus 30 of the central conduit 14 in the chamber 32. Residence times are controlled by the nozzle geometry, location, and gas flow rates, and can be as short as a few milliseconds such as 0.001 seconds or as long as about 10 seconds.
The temperature at the terminus 30 is about 11,000 K. The high temperature is generated by the RF pulsing of the induction coils 24, ionizing the plasma gas within the reactor 10 volume. Temperatures can be adjusted from about 3,000 to 11,000 K.
Ultrafine (or nanosized) metal powders 36 are ejected from the exit nozzle 34 of the torch 10 into a reactor (not shown) where they are treated and then collected after passing through filters, such as sintered metal filter elements and other equipment known to those in the art.
As the metal carbonyl rapidly dissociates and the pure metal is quenched, the resulting homogenous nucleation gives rise to a very fine aerosol. The particle size distribution and crystal structure of the nanopowder are functions of the aerosol quench rate, the type of quench gas and precursor metal carbonyl gas composition or concentration. Typically inert quench gases such as argon or nitrogen are used for pure metal powder production. Reactive quench gases such as oxygen, ammonia or methane allow for the synthesis of ultrafine oxides, nitrides or carbides.
Regarding plasma energy, in a typical 64 kW torch, the energy coupling efficiency is about 65%, and the “overall” efficiency (taking into account all cooling and heat losses and coupling efficiency) is 30%, leaving about 19 kW net available power in the plasma. Only part of this is used for the dissociation of carbonyl (the rest of the energy essentially heats the gases and the resultant metal powder product) thus giving the final overall process efficiency of about 14%. A 64 kW plasma unit is expected to produce about 5 kg of metal nanopowder per hour.
A series of prototype tests were conducted to assess the efficacy of the present invention.
A Tekna Plasma Systems Inc. PL-50 induction plasma torch was employed along with a subsequent cyclone and filter baghouse to retrieve the metal powders. Torch plate power was 24-65 kW. The sheath gas was a helium/argon mixture delivered at 401/min/1001/min and 12 psia (82.7/kPa). Nickel carbonyl and the carrier gas of helium and carbon monoxide in a 20:1 ratio were delivered at 201/min and 0-5 psig (34.5 kPa).
Test results are shown below in Tables 1 and 2.
Metal carbonyl gas 40 and a carrier gas 42 such as helium and carbonyl monoxide are introduced into induction plasma torch 44. Plasma gas 46 typically argon and sheath gas 48, typically argon and hydrogen, are supplied to the torch 44.
Upon emerging from the torch 44, the ultrafine metal is treated with a quench gas 50, typically argon and nitrogen in reactor 52 to cool the particles and, if desired, to react with the particles.
Upon sufficient cooling, the particles are routed to a filter 54 which may be, for example a cyclone and/or bag house. The finished product is collected in a container 56.
Remaining processing and product gases are separated at a stage one separator 58. The processing gases, primarily the carrier gas, plasma gas, sheath gas and quench gas are routed to stage two separator 60 for subsequent treatment. Carbon monoxide, the primary gaseous by-product of the dissolution reaction in the torch 44, is routed to a catalytic converter 62 where it is split into carbon and oxygen or oxidized to CO2 and removed as an off gas 64. Air 66 is supplied as necessary. Alternatively, the carbon monoxide may be recycled for additional metal carbonyl production.
Although primarily addressed to nickel nanopowder production, the present invention is applicable to any metal carbonyl, such as iron, copper, cobalt, chromium, molybdenum, tungsten, and ruthenium. Moreover, both gaseous and liquid forms of the metal carbonyl may be introduced into the torch 10.
While in accordance with the provisions of the statute, there is illustrated and described herein specific embodiments of the invention. Those skilled in the art will understand that changes may be made in the form of the invention covered by the claims and that certain features of the invention may sometimes be used to advantage without a corresponding use of the other features.
