Luminous discharge cell for spectrographic analysis

[57] ABSTRACT

The cell has an anode with a light-transmitting through bore, one end of which is directed towards a sample-supporting cathode, the other end being directed towards a window in an anode housing. An electrically insulating sleeve is interposed between the anode and the housing. The sleeve is slidable along the anode and surrounds the end of the anode which faces the cathode. The annular end of the sleeve can be applied against a seating surface of the sample support.

Claims, 7 Drawing Figures

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U.S. Patent Sept. 30,1975

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U.S. Patent sept. 30,1975

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U.S. Patent sept. 30,1975

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3,909,652

LUMINOUS DISCHARGE CELL FOR SPECTROGRAPHIC ANALYSIS

The present invention concerns a luminous discharge cell for spectrographic analysis.

The invention relates especially to a cell for analysis in a rarefied gas, comprising a sample support; a sealed chamber which is defined by walls and into which open a supply conduit for gas and a pumping conduit; a cathode which is connected to the negative pole of an electric generator; an anode which is connected to the positive pole of the electric generator and which has at least one bore of rectilinear axis which passes through it from one end to the other and which opens on the one hand through an end, face of the anode towards the sample to be analysed, and on the other hand through the opposite face of the anode towards a window; and means for applying the sample support sealingly. against a support wall of the cell.

In a known, cell of this type the cathode is hollow and constitutes the support wall of the cell. The support for the sample to be analysed, which constitutes the cathode bottom is applied sealingly to the support wall by means of a toroidal seal. The body of this known cell is provided with a tip penetrating into the body of the cathode.

By reason of the design and the geometry of this known cell, metallic beads tend to form by reason of the electrical discharge, mainly, radial, set up between anode and cathode, with the risk that short circuits will be produced, rendering the results of analysis unreliable and thus limiting the performance of the spectrographic apparatus.

Moreover, in this known cell, the presence of the toroidal seal between the sample support and the support wall constituting the cathode means that the electrical contact between the sample support and the wall is not complete. To produce complete contact it would be necessary to use a strong spring thrusting the sample support against the cathode wall, which would mean unwanted crushing of the seal. Moreover, it the sample support has a bent shape, in order to produce effective sealing between the sample support and the support surface and to produce good electrical contact, it is necessary to use intermediate pieces of complicated shape with toroidal seals of suitable shape. As a result the use of this known cell is difficult and limited.

The object of the present invention is a spectrographic analysis cell eliminating or at least reducing undesired electrical discharge between anode and cathode and allowing a wide range of qualitative and quantitative spectrographic analyses to be carried out on a variety of materials, with low consumption of the materials.

In accordance with the invention, the sample support is connected to the negative pole of the electric generator by conductive means separate from the support wall of the cell, but with interposition, between the end of theanode adjacent the cathode and an adjacent wall of the cell, of an insulating sleeve which surrounds the said end of the anode which can slide along the anode, and which can be applied by an anriular face against the face, turned towards the anode, of the sample support.

Thus in this cell the end of the insulating sleeve rests on the cathode, which may or may not be flat and. which constitutes the support for the sample to be analysed, with the result that this insulating sleeve pre cisely defines the area with which the analysis is concerned. The insulating sleeve ensures sealed connection against electrical discharge, on the one hand with the sample support and on the other hand with the body of the anode in such a manner as to define, with the internal face of the bore of the anode and the inner face of the sample support, the cell chamber.

In a preferred embodiment of the cell gas passes through the bore of the anode and returns through axial passages provided along at least a portion of the length of the anode. Such axial passages are for instance made by machining the external surface of the anode.

Other characteristics and advantages of the invention will be better understood from the following description of several embodiments of the invention, given by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 is a view in axial section of one embodiment of luminous discharge cell;

FIG. 2 is a view, on a larger scale, in cross section of the anode of the cell shown in FIG. 1;

FIG. 3 is a part view in axial sectiort'of another embodiment of cell;

FIG. 4 is a view in axial section of another embodiment of cell;

FIG. 5 is an exploded sectional view of the insulating sleeve, the sample support, and the electrical connection rod of the cell shown in FIG. 1;

FIG. 6 is an exploded sectional view of an alternative embodiment of the insulating sleeve, the sample carrier, and the electrical connection rod; and

FIG. 7 is an exploded sectional view of a further alternative embodiment of the insulating sleeve, the sample carrier, and the connection rod.

FIG. 1 shows a luminous discharge cell for spectrographic analysis having a sealed cell chamber 1, into which opens a supply conduit 2 for a neutral gas such as argon and a pumping conduit 3 connected to a pumping unit (not shown). The cell has a cathode connected to the negative pole of an electric generator (not shown) and an anode 4 connected to the positive pole of the electric generator. The anode 4 is constituted by a metal tube with a rectilinear bore 4a which extends through it from one end to the other; the bore 4a opens out at one end towards a support 5 for the sample to be analysed, and at the other end towards a transparent window 6.

The cell shown also has means for applying the internal face 5a of the sample support 5 sealingly against the annular sealing face Ία of an annular metal support wall 7 electrically insulated from the anode 4, in such a manner that the sample support 5 defines the cell chamber 1. The annular support wall 7 is cooled by water circulation, symbolised in FIG. 1 by the arrow f,.

The cathode of the cell is constituted by the sample support 5 alone, which is connected to the negative pole of the electric generator by means of conductors separate from the annular support wall 7 of the cell.

Moreover, between the end of the anode 4 and the support wall 7 is interposed a continuous insulating annular wall or sleeve 8, which is fitted on and surrounds the end of the anode 4, and is mounted on the anode so as to be slidable axially parallel to the axis of the bore 4α of the anode 4; the entire annular end face 8α of the sleeve 8 is applied against the internal face 5α of the sample support 5. The sleeve 8 is easily replaceable.

3,909,652

The annular support wall 7 is housed and fixed in a cell body 9 of‘‘Plexiglass (a Trade Mark for methyl methacrylate polymer) or another insulating material. In the example of FIG. 1, where the wall 7 is of metal, it has an insulating envelope 10, for example of Teflon” (a Trade Mark for polytetrafluoroethylene). In the interior of the envelope 10, which is for instance fitted tightly in an axial bore of the wall 7, the sleeve 8 can slide axially. The sleeve 8 can be made of quartz or another electrically insulating material of good thermal 10 behaviour.

A compression spring 11 is interposed between the end of the insulating sleeve 8 and a shoulder provided on the anode 4 and is arranged to thrust the insulating sleeve 8 against the sample support 5. The spring also 15 facilitates the extraction of the sleeve 8.

The anode 4, which is made for instance of Copper, is mounted in the cell so as to be axially displaceable. The anode is screw threadedly engaged with a support 14. Displacement of the anode is effected by means of 20 a worm 13 and worm-wheel 12 rigid with the anode 4. Rotation of the worm 13 on its spindle 13α causes rotation of the screw-threaded anode 4 in its support 14 and thus brings about the axial displacement of the anode. The spindle 13α extends outside the cell, thus 25 making it possible to effect from the outside the regulation of the axial position of the anode 4, and consequently the spacing between the anode 4 and the cathode (sample support 5).

The anode 4 and the sleeve 8 can easily be disman- ³⁰tied and replaced by similar members having different dimensions. It is advantageous for the sleeve 8 to be long enough to permit spacings of some tenths of a millimetre to several centimetres between the anode 4 and the sample 5. It is thus possible, by suitably regulating ³⁵this spacing, to achieve the best conditions for analysis by limiting the vapourisation of the sample between the two electrodes.

The window 6 is either a plate with parallel faces (a quartz plate in the example shown) or a non-absorbent lens; its purpose is to allow the luminous radiation from the sample to pass a spectrograph (not shown).

It will be observed that the possibilities of regulation offered allow various exposure times, in particular extended exposure times and also the use of different current strengths. Such possibilities ensure great exploitation of spectrometric measurement apparatus used for analysing the light emitted by the cell.

The gas, for instance argon, is introduced into the ₅θ cell through the conduit 2, passes along the axial bore ³4α and then returns between the anode 4 and the sleeve 8 through channels made either in the external face of the anode 4, as in the case of the channels 30 shown in FIG. 2, or in the internal face of the sleeve 8, dr through any other suitable cavities, the anode 4 being fitted with play in the sleeve 8. The gas pressure in the cell chamber 1 is maintained at a suitable value, between 2 mm and 30 mm of mercury, by means of a pump connected to the conduit 3. Gas-tight sealing between the sample support 5 and the sealing face 7α is effected by means of a toroidal seal 15.

The anode support 14 is cooled by water circulation, symbolised in FIG. 1 by the arrow /). The supply of electrical current to the anode is effected by means of the support 14, through a terminal 14b.

The sample support 5 in the embodiment shown comprises a copper ring 5b enclosing a mass of conduc4 five powder 5c (e.g. graphite) the surface Sa of which supports the particles to be analysed (not shown).

A centering and cooling member 18 is used, in particular when the sample support 5 is flexible or has small 5 dimensions. If it is rigid a spring 16 will be sufficient to apply it against the wall 7. Cooling is ensured by water circulation / in the member 18.

FIG. 3 shows an embodiment of cell particularly adapted for the study of curved surfaces. A washer or intermediate piece 19 is fitted on one end of the insulating sleeve 8 and is applied sealingly by means of toroidal seals 20, 21 respectively against the sealing face 7α of the support wall 7 and against the face 5α of the sample support 5.

The discharge cell described above, in view of the regulating possibilities, the interchangeability, the ease of access, and the easy cleaning of all its constituted parts, can be adapted to numerous analytical problems. In particular, as concerns the analysis of solids, it can be used in the examination of finished flat or curved pieces, or permits the examination of surface layers of materials. It can even be used in the analysis of liquids (impregnated cathode) or gases; in the latter case the circulation of the gases takes place either from the anodic compartment or from the cathode. It is also possible to analyse gases extracted in the course of cathodic bombardment. Finally, an important application is provided by the possibility, by virtue of the regulation limiting vapourisation, of effecting several quantitative analyses on the same particle of small dimensions, in particular on particles collected in the oil filters of machines.

In fact, the wear on rotating pieces in an engine manifests itself inter alia by the transference to the lubricating oil of metal particles of very different sizes and shapes; some of these remain in suspension in the oil, others may be retained by filters arranged in the oil circuits.

It is of particular interest in aviation to follow the development of wear phenomena in engines by the analysis of solid particles carried along by the oil in operation. The analysis of these particles and the knowledge of the constituent materials greatly facilitate the pinpointing of damage. The analysis is carried out after the particle has been coated by means of a conductive powder forming an electrically conductive disc used as cathode. An automatic disc manipulation device can be produced for presenting particles successively for analysis.

Finally, because of the possibility of carrying out non-destructive analyses, the cell can also find application in checking the nature of the surface of finished pieces.

Alloys as well as non-metallic materials may contain a very small quantity (0.1 p.p.m. to 1 p.p.m.) of metallurgical components or impurities, and it is difficult to determine these by known methods.

The luminous discharge cell forming the subject of the invention has particularly interesting application when it is a question of determining volatile elements in a refractory-based alloy. Alloys on a basis of nickel, for instance, can be contaminated by bismuth, tin, lead, or silver.

The bismuth may notably come from a material (lowmelting-point alloy) used for coating casting in order to facilitate their machining, and should not exceed 1 p.p.m. One of these materials, used commercially

3,909,652 under the name of CERROBAND, is used in particular in operations of pinning the feet of vanes.

The embodiment described below with reference to FIGS. 4 to 7 is an adaptation of the luminous discharge cell described above, to enable analysis of these volatile elements to be undertaken with great ease.

For this purpose, a hollow sample carrier 50 is fitted in the end of the insulating sleeve 8 and opens into the sleeve 8 through an orifice 50y coaxial with the sleeve. The orifice 50/ has a cross-sectional area less than that of the mid-section of the cavity of the sample carrier 50.

Accordingly, in this cell, the volatile elements are confined in the cavity of the sample carrier and the analysis carried out is more accurate.

FIG. 4 also shows a sealed cell chamber 1 into which opens a supply conduit (not shown) for a neutral gas, such as argon and a pumping conduit (not shown) connected to a pumping unit. The cell has a cathode connected to the negative pole of an electric generator (not shown) and an anode connected to the positive pole of the generator. The anode is constituted by a metal tube 4 having a rectilinear bore 4α which passes through it from one end to the other; one end faces the sample carrier 50 containing the sample 50m to be analysed and the other end faces a transparent window (not shown).

The sealed non-conductive chamber 1, for instance of “PYREX” (a Trade Mark for methyl methacrylate polymer) is defined on the sample side by a cooled wall 18, which carries and surrounds the sample carrier 50 and whose free edge 18α is applied sealingly against an adjacent cooled annular support wall 7 of the cell. The sample carrier 50 and the wall 18 together constitute the sample support of the cell.

Moreover, between the support wall 7 and the end of the anode adjacent the sample carrier is interposed an insulating annular wall or sleeve 8, which is fitted on and surrounds the said end of the anode 4. The sleeve 8 is axially slidably mounted on the anode 4, and its annular end face 8α (preferably the entire extent of this face) is applied against a shoulder 50&lt;? on the sample carrier 50.

The wall 7 is of metal and has an insulating envelope 10, of Teflon for instance. In the interior of the envelope 10, which is for instance a force fit in an axial bore of the wall 7, the sleeve 8 can slide axially. This sleeve is easily interchangeable and can be made of quartz or another electrically insulating material of good thermal behaviour.

A compression spring (not shown) can be interposed between the insulating sleeve 8 and a shoulder (not shown) on the anode 4 so as to thrust the sleeve 8 against the sample carrier 50.

The anode 4, for instance of copper, is mounted in the cell so as to be axially displaceable, in the manner described above, for example, so that axial displacement can be effected from outside the cell, to regulate the axial position of the anode 4 and consequently of the sample carrier 50.

The anode 4 and the sleeve 8 can easily be dismantled and replaced by similar members of different dimensions. It is advantageous for the sleeve 8 to be of such length that the spacing between anode 4 and cathode sample carrier 50 is some tenths of a millimetre to several centimetres.

The support wall 7 has been modified to receive the cooled wail 18 of PYREX, the sealing being effected by means of a toroidal seal 100.

In the embodiment of FIG. 5 the sample carrier 50 is hollow and is constituted of two parts 50α and 50ά made of graphite. The part 50ά has a bore 50q which receives with slight play a conductive rod 16 made of tungsten, and has a recess 50ά intended to contain the sample to be analysed, which is generally in the form of powder or chips.

The end 50ά of the part 50 ά fits with slight play in a bore 50t· made in the part 50α, which is itself fitted in the sleeve 8. The cross-section of the orifice 50J of the part 50α is smaller than that of the recess 50ά in the part 50ά.

FIGS. 6 and 7 show, in exploded view, similar to the exploded view of FIG. 2, two variants of assembly of the sample carrier 50.

The sample 50wt to be analysed is arranged in the interior of the hollow sample carrier 50 which is then fitted in the end of the insulating sleeve 8. The cavity of the carrier communicates with the interior of the sleeve 8 through the orifice 50/, 50/', coaxial with the sleeve 8 and having a cross-section less than the average cross-section of the 50ά, 50ά' of the sample carrier 50.

In order to carry out the analysis of the volatile elements, one focuses in the outlet plane 50/ of the sample carrier 50.

The base metal of the sample, for instance the nickel, is generally less volatile and makes only slight appearance in the analysis.

The cell described above has notably the following advantages:

one can reuse the electrode after the sample carrier and the sleeve have been cleaned;

the electrical discharge is channelled parallel to the axis and can thus be concentrated on the sample to be analysed;

the volatile elements are confined in the graphite sample-chamber constituted by the part 50ά and analysis is sensitive;

the cell is easier to use than known cells, since only a few minutes are needed for changing or cleaning the sample carrier 50.