US3547802A - Low temperature r-f reactor - Google Patents

Description

15,1970 umran 3,541,802-

LOW TEMPERATURE R-F REACTOR Filed Aug. 12, 1968 United States Patent Ofiice 3,547,802 Patented Dec. 15, 1970 US. Cl. 204312 Claims ABSTRACT OF THE DISCLOSURE A low temperature radio frequency reactor Whose reaction chamber has a compact onfiguration. The configuration includes a linear, axially symmetrical reaction portion, and smoothly curved end portions. The folded reaction chamber has an efiiciency nearly that of a greatly elongated chamber but is more practical, compact, and emits less radiation.

BACKGROUND OF THE INVENTION The present invention relates to a plasma reaction device. More particularly, the invention relates to a compact plasma reaction device whose reaction chamber has a configuration for practical and efficie-nt operation.

There is frequently a need for a device which will promote chemical changes at relatively low temperatures in a sample. For example, in an analytical technique known as ashing, it is desired to remove components (carbon, hydrogen and nitrogen) of organic materials from samples to clear the field for analysis of metals or mineral constituents. Moreover, samples to be analyzed for mineral microstructures must not be physically disturbed while organic materials are removed from the skeletal substructures to reveal the inorganic matrix. One such device is the low temperature radio frequency reactor.

A low temperature radio frequency reactor is a device in which a plasma is formed by coupling the output of a radio frequency oscillator to a stream of a reaction gas at a low pressure. The oscillator may operate at the Federal Communications Commission assigned frequency of 13.56 mHz. The gas may be at a pressure of not more than about 10 mm. of mercury, absolute. Chemically active neutral species are then produced in the low pressure gas confined in a high frequency electric field through the collision of accelerated ions and electrons with neutral molecules. These active neutral species are the active agents in the plasma and they are capable of promoting chemical changes at relatively low temperatures in a sample under test.

Although the general principle for a low temperature radio frequency reactor is known, there is only one form of such a device available commercially today. This prior art device consists of an L-shaped chamber in which one leg of the L is used for the generation of the plasma and the other leg of the L is used for reacting the plasma with the sample. This prior art device is characterized by, inter alia, sudden changes in the size of the cross-sectional area of the plasma path, sharp changes in the direction of plasma flow, relatively short arms of the reaction chamber, and chambers terminating with abrupt end walls. It has now been found that all these features are detrimental to the eflicient plasma generation and control. In addition, unless a ground plate is placed below the sample in this prior art device, which would further lower the efficiency thereof, the plasma would not be directed to the sample and it may rapidly pass over the sample and out of the device, thus leaving the sample substantially unchanged. Moreover, the loss of volatile compounds, as distinguished from the organic compounds in the sample, would be relatively high in such a device. It is accordingly an object of the present invention to provide a novel low temperature radio frequency reactor.

It is another object of the invention to provide an efficient and compact radio frequency reactor which can be practically utilized in any laboratory.

These and other objects of the invention will become apparent from a reading of the disclosure herein,

SUMMARY OF THE INVENTION In accordance with the present invention, a compact and yet eflicient radio frequency reactor is provided which has a plasma reaction chamber in which the plasma generating section and the sample containing section are located in a linear portion of the chamber having substantially uniform cross-sectional configuration and axial symmetry. The terminal ends of the chambers are curved in a manner to avoid undesirable abrupt end walls. Such a device has an efilciency nearly approaching that of an ideal, elongated, straight chamber but it is more compact and can be conveniently used in any analytical laboratory.

As indicated above, it is known to excite a stream of gas, such as hydrogen, oxygen or helium, confined at low pressure in a high frequency electric field. Chemically active neutral species are then produced in this field through the collision of accelerated ions and electrons with neutral molecules. Such chemically active neutral species are the active agents in the radio frequency reactor for bringing about chemical changes at relatively low temperatures in the sample.

Recombination or destruction of the active species so generated can occur either in the bulk of the gas or plasma, or at the walls of the containing vessel. Since destruction of the active species renders the plasma less effective, it is to be avoided. Recombination of the active species in the bulk of the plasma can be suppressed or reduced by maintaining the pressure of the gas stream to less than 10 mm. of mercury. The rate of destruction of the active species at the walls of the containing vessel is directly dependent on the rate at which active species reach the walls and the wall temperature. It can be shown that the radial concentration gradient of electrically charged particles Within a cylindrical vessel surrounded by a concentric electric field can be described by a zero-order Bessel function with concentrations approaching zero at the vessels surface. Due to the rapidity of exchange reactions, and due to the fact that neutral active species are generated by the collisions of charged species with neutral molecules, the radial concentration gradient of the neutral active species follows approximately that for the charged particles. The present invention takes advantage of this fact: that the concentration of the neutral active species, under the influence of the electric field of the high frequency oscillator, is very low at the vessels surface unless these neutral active species are forced to impinge the container walls. The device of the invention maintains a low concentration of the neutral active species at the walls of the reaction chamber by, as much as possible, not disturbing the plasma flow in the chamber. This is accomplished, in the present invention, by having a reaction chamber in which the plasma generating section and the sample containing section are in a linear portion of the chamber, which :portion is also axially symmetrical in the longitudinal direction. In addition, to further eliminate disturbance of the plasma flow, said portion of the chamber is made to have a substantially uniform cross-sectional area throughout. This further eliminates the end-plate effect.

The symmetrical nature of the reaction portion of the chamber, which contains the plasma generating section and the sample containing section, also assists in the reduction of loss of neutral active species. When an asymmetrical arrangement is used, there is present a component of the field which drives high velocity electrons and ions against the walls of the vessel. This leads to both deterioration of the equipment and heating of the vessels walls. Since the loss of neutral active species is substantially greater at higher temperatures, the asymmetrical arrangement of the prior art device further contributes to a low overall efiiciency. This defect is also corrected by the construction in the device of the present invention.

To reduce the size of the radio frequency reactor to a compact and manageable proportion, the terminal ends of the reaction chamber are curved and folded into smoothly curved sections. Preferably these folded terminal sections should also have substantially uniform crosssectional areas corresponding to that of the reaction portion of the chamber. These terminal portions of the re action chambers can be made to be detachable from the linear portion of the chamber so that samples can be inserted and withdrawn from the linear portion of the chamber and that the chamber can be easily cleaned or decontaminated.

BRIEF DESCRIPTION OF THE DRAWING AND THE PREFERRED EMBODIMENT The invention will now be described with reference to the drawing which shows one embodiment of the device of the invention in perspective. In this drawing, a reaction chamber for a low temperature radio frequency reactor is generally shown at 10. At the entrance portion 11 of the reaction chamber there is a fish-hook type trap 12 which communicates with a supply (not shown) of the reaction gas to be used. The fish-hook trap permits the entry of the reaction gas into the reaction chamber but it effectively prevents the backfiow of plasma which is detrimental to the equipment. The entrance section 11 is smoothly curved for the reasons disclosed herein. The outlet end of entrance section 11 is connected through a taper joint 13 to the reaction portion 14 of the reaction chamber. A plasma generating coil 15 surrounds the reaction portion 14 of the chamber near its entrance. Coil 15 is the output end of a radio frequency oscillator (not shown) and the coil is coaxial with the reaction portion 14 of the chamber for maximum effectiveness. Alternatively capacitive plates or other coupling device may be substituted for coil 15. A sample containing region 16 is located in the reaction portion 14 of the chamber 011 the downstream side of the coil 15. We have found that the optimum location of the sample containing region 16, for the most efiicient utilization of the plasma generated by coil 15, is at a point at or near the longitudinal axis of the reaction portion 14 of the chamber, and a few centimeters downstream from the end of coil 15. If the sample containing region is located at the upstream end of coil 15, we have found that the efiiciency would be lowered by about The sample may be positioned in the sample containing region 16 by a sample boat or other appropriate devices (not shown) located near the axis of the chamber.

The outlet end of the reaction portion 14 of the chamber is connected to a U-shaped exit portion 17 of the chamber through a ball joint 18. Exit portion 17 is made to have substantially the same cross-sectional area and cross-sectional configuration as those of the reaction por tion 14 of the chamber so that there will be no abrupt changes in the size of the plasma path and no sharp changes in direction of the plasma flow. In addition, the U-shaped exit portion 17 is designed to avoid the so-called end wall efiect which would reduce the effectiveness of the plasma at the sample containing region 16. The exit portion 17 of the chamber is connected to a cold trap 19 through a ball joint 20. Cold trap 19 is in the form of a conventional finger trap and it serves the dual functions of (l) preventing the plasma flow from traveling further downstream which would be damaging to the equipment thereat, and (2) retaining the reaction products between the plasma and the sample. The cold trap 19 is connected to a vacuum pump (not shown) via a conduit 21. The vacuum pump maintains the proper pressure within the reaction chamber during operation.

It will be appreciated from the drawing that the fishhook trap 12 serves the purpose of admitting the reacting gas into the reaction chamber while at the same time preventing any back flow of the plasma to the damage of equipment. Modifications of this trap can be effected to accomplish the purpose. The joint 13 between entrance portion 11 and reaction portion 14 of the chamber shown in the drawing is a taper joint. The joint 18 between reaction portion 14 of the chamber and the exit portion 17 of the chamber is a ball joint. These joints are chosen for their convenience in assembly, freedom from sudden diameter changes or sealing compound exposure, and relative immunity to misalignment. The joint between exit portion 17 of the chamber and the cold trap 19 is also a ball joint so that the exit portion 17 can be easily removed for insertion of sample into the sample containing region 16.

When the device shown in the drawing is to be used for dry ashing of a sample, the exit portion 17 is first disconnected from the device at the ball joints 18 and 20. The sample is then inserted into the sample containing region 16 and the exit portion 17 reconnected. The reaction gas, such as oxygen, is supplied from a container to the reaction chamber via fish-hook trap 12. When the reaction gas is under the influence of the electric field generated by coil 15, a plasma containing neutral active species is formed which comes into contact with the sample at region 16. After contacting the sample, the plasma is passed on to exit portion 17 of the chamber. The sudden change in the cross-sectional area encountered by the plasma at the outlet end of the exit portion 17 causes destruction of many of the neutral active species. However, this sudden change in the chamber configuration takes place sufficiently far downstream as to have substantially no effect on the quality of the plasma in contact with the sample at region 16. After the reaction products have been collected by the cold trap 19, the remaining gas passes out of the reactor with the assistance of a vacuum pump.

As indicated above, the R-F reactor of the invention is useful in preparing samples for analysis. In addition, the reactor can be used to alter the molecular structures of plastic surfaces. When metallic surfaces are to be examined, these surfaces can be cleaned of oxides such as ferrous oxide by the use of a stream of hydrogen in the R-F reactor as the reaction gas to reduce the oxide to its metallic state. The hydrogen combines with the oxygen in the oxide to yield an atomically clean surface for examination without loss of sample and without contamination of the surface.

It will be noted that the radio frequency reactor of the present invention has a reaction chamber with a configuration which takes up a minimum of space and yet avoids the adverse defects of sharp changes in direction of the plasma flow, sudden changes in cross-sectional area of the plasma path (right angled impingement of plasma on th sample), bypassing of the sample by the plasma, and abrupt end Walls in the plasma path. When compared to an ideal radio frequency reactor having a linear tube extending over several feet in length, the curved and folded reactor chamber of the present invention yields an efficiency about of the ideal device. Thus, there is substantially no sacrifice in the efficiency of the reactor.

The advantages derived from the present design of the radio frequency reactor are that valuable laboratory space can be saved; the device can be moved and transported much more easily; the device can be more easily taken apart and decontaminated; the insertion of sample is made much easier; and the curved or folded design of the reaction chamber reduces radiation loss from the system which greatly eases the problem of shielding the system to conform with the Federal Communication Commission requirements with respect to the shielding of radio frequency generating devices. The last mentioned advantage is particularly important since the device must pass a series of Federal Communication Commission tests for certification. A commercial form of the device as illustrated in the drawing recently passed all such tests for certification due to its low radiation loss. The low radiation loss from the device is in major part due to the folded configuration of the terminal ends of the reaction chamber, which folding substantially cancels the radiation loss from the device. Because of this cancelling out etfect, the shielding of the device becomes relatively simple. A perforated shield can be used to etfectively reduce all radiation at 1,000 feet to less than ambient noise.

The low radiation level made possible by the folded configuration of the present device further facilitates the visual observation of the reactor when it is in operation. If the device must be extensively shielded, then it would be extremely difficult for an operator of the device to observe the sample during the reaction process. However, since the present device can be shielded by a per forated (enclosure, an operator of the device can observe the sample through the meshed shield.

The invention has been described with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope and spirit of the invention as described hereinabove and as defined in the appended claims.

What is claimed is:

1. A compact plasma reaction device comprising: A plasma reaction chamber axially symmetrical in its longitudinal direction and having a central linear portion of substantially uniform cross sectional configuration, a curved entrance portion for admitting an ionizable gas and a curved exit portion, said central portion having a plasma generating section and a sample-containing section downstream of said generating section, said entrance and exit portions being joined to said central portion in a manner to avoid sharp change in direction for plasma fiow therethrough and to minimize radiation loss from said device, and a means external of said chamber for supporting a R-F field of sufiicient intensity to ionize the gas within said generating section, whereby the overall length of said chamber and the radiation loss from said chamber are minimized without any substantial adverse effect on the quality of the plasma generated therein.

2. A device according to claim 1 wherein said entrance portion is curved to form a smooth 90 angle and said exit portion is curved in a smooth U shape.

3. A device according to claim 1 further comprising means attached to the free end of said entrance portion for introducing a reaction gas into said chamber but preventing back-flow of said plasma.

4. A device according to claim 1 further comprising a means attached to the free end of said exit portion for retaining reaction products and serving as a termination of said plasma.

5. A device according to claim 1 wherein said chamber includes means to permit the introduction of a sample into said sample containing section.

6. A device according to claim 5 wherein said means permitting the introduction of a sample is said exit portion, said exit portion being curved in a smooth U shape and detachable from said chamber for the introduction of the sample.

7. A device according to claim 1 wherein said sample containing section has means for positioning said sample at about the longitudinal axis of said chamber.

8. A device according to claim 1 wherein said plasma generating means is a radio frequency means.

9. A compact plasma reaction device comprising a plasma reaction chamber axially symmetrical in its longitudinal direction and having a central linear portion without abrupt changes in cross sectional configuration, said central portion terminating at its ends in an entrance portion for admitting an ionizable gas and an exit portion and having a plasma generating section and a sampie-containing section downstream of said generating section, at least one of said entrance and exit portions being curved to avoid sharp change in direction for plasma flow therethrough and to minimize radiation loss from said device, and a means external of said chamber for sup porting a R-F field of sufficient intensity to ionize the gas within said generating section, whereby the overall length of said chamber and the radiation loss from said chamber are minimized without any substantial adverse effect on the quality of the plasma generated therein.

10. A compact plasma reaction device comprising: a plasma reaction chamber made of glass and axially symmetrical in its longitudinal direction, said chamber having a central linear portion of substantially uniform cross sectional configuration, an entrance portion smoothly curved to form a 90 angle and joined to said central portion through a bell joint, for admitting an ionizable gas an an exit portion smoothly curved to form a U shape and joined to said central portion through a ball-andsocket joint, said central portion having a plasma generating section and a sample-containing section downstream of said generating section; a trap means attached to the free end of said entrance portion for introducing a reaction gas into said chamber but preventing back-flow of said plasma; a trap means attached to the free end of said exit portion through a ball-and-socket joint for retaining reaction products and serving as a termination of said plasma; said exit portion being detachable from said central portion for ingress and egress of samples from the central portion; and a means external of said chamber for supporting a R-F field of suflicient intensity to ionize the gas within said generating section, whereby the overall length of said chamber and the radiation loss from said chamber are minimized without any substantial adverse effect on the quality of the plasma generated therein.

References Cited UNITED STATES PATENTS 3,410,776 11/1968 Bersin 204-193 3,428,548 2/1969 Hollahan 204-312 3,049,488 8/1962 Jackson et al. 204312 HOWARD S. WILLIAMS, Primary Examiner N. A. KAPLAN, Assistant Examiner U.S. Cl. X.R. 204l64

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