US3899627A

US3899627A - Crucible

Info

Publication number: US3899627A
Application number: US484303A
Authority: US
Inventors: George J Sitek; Charles W Berk
Original assignee: Leco Corp
Current assignee: Leco Corp
Priority date: 1974-06-28
Filing date: 1974-06-28
Publication date: 1975-08-12
Anticipated expiration: 1992-08-12

Abstract

A resistance heating crucible for use in holding a sample to be fused by a furnace includes a cylindrical side wall communicating with a bottom floor having an exterior button-like projection extending centrally therefrom to define an annular contact surface surrounding the button-like projection for seating with the crucible-activating electrode associated with the furnace. The circular bottom is chamfered along the junction of the bottom with the cylindrical side wall to reduce and limit the annular contact surface. the interior floor of the crucible is rounded to reduce the cross section through the bottom and in cooperation with the chamfered edge, increases the heating effect of current passing through the bottom of the crucible.

Description

. United States Patent [191 I Sitek et a1.

[ CRUCIBLE [75] lnventors: George J. Sitek, Stevensville;

Charles W. Berk, St. Joseph, both of Mich.

[73] Assignee: Leco Corporation, St. Joseph, Mich.

[22] Filed: June 28, 1974 [21] Appl. No.: 484,303

[ 1 Aug. 12, 1975 Primary Examiner-R. N. Envall, Jr. Attorney, Agent, or Firm-Price, l-leneveld, l-luizenga & Cooper [5 7 1 ABSTRACT A resistance heating crucible for use in holding a sample to be fused by a furnace includes a cylindrical side wall communicating with a bottom floor having an exterior button-like projection extending centrally therefrom to define an annular contact surface surrounding the button-like projection for seating with the crucible-activating electrode associated with the furnace. The circular bottom is chamfered along the junction of the bottom with the cylindrical side wall to reduce and limit the annular contact surface. the interior floor of the crucible is rounded to reduce the cross section through the bottom and in cooperation with the chamfered edge, increases the heating effect of current passing through the bottom of the crucible.

10 Claims, 4 Drawing Figures CRUCIBLE BACKGROUND OF THE INVENTION The present invention relates to crucibles for fusion of a specimen and particularly to crucibles made of a resistive material.

In resistance furnaces for fusing specimens to obtain gas samples therefrom of the type manufactured by Leco Corporation of St. Joseph, Michigan, as an integral part of an analyzer, Model No. TC-30, graphite specimen-holding crucibles are employed for containing the sample. The crucible is positioned between actuating electrodes for heating the sample to a fusion temperature. Several crucible designs have been previously employed in such furnaces including a stud crucible of the type described in U.S. Pat. No. 3,636,229 assigned to the present assignee.

Although prior crucibles have worked well for certain specimens such as oxygen and adequately for other specimens, it has been discovered that with some specimen gases such as nitrogen which require significant energy to cause the required reduction reaction with the carbon crucible, existing crucibles have not performed as well as desired.

SUMMARY OF THE INVENTION It was discovered that the crucible bottom can be configurated to reduce the electrical current crosssectional area thereby increasing the direct heating of the bottom instead of through conduction of a heated stud as disclosed in the above identified patent. The increased efficiency in directly heating the crucible floor for fusing the specimen permitted the improved crucible design disclosed herein to be employed with the same furnace having the same maximum currents as previously used but with considerably more success with specimen gases such as nitrogen. Thus, by virtue of the improved crucible design, for the same amount of applied power, significantly greater efficiency of fusion of the sample is provided.

The crucible design of the present invention also increases the dispersion of the molten specimen during fusion to further reduce the specimen as it reacts with the carbon crucible. Also, reduced furnace power can be employed with many specimens. Finally, in some cases, fluxes previously required to achieve complete fusion can be eliminated.

Crucibles embodying the present invention include a cylindrical side wall communicating with an integral bottom. The junction between the side wall and the outside surface of the crucible bottom is chamfered to reduce the contact area between the crucible bottom and the furnace electrode supporting the crucible. The floor of the crucible bottom is rounded which, in cooperation with the chamfered side walls, reduces the cross-sectional area of the current path through the bottom of the crucible thereby increasing the heating of the bottom by current flowing therethrough. In one embodiment, the bottom includes an exterior button- BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a fragmentary cross-sectional view of a resistance heating furnace and the crucible embodying the present invention shown positioned in relation to the furnace electrodes;

FIG. 2 is an enlarged cross-sectional view of the crucible shown in FIG. 1;

FIG. 3 is a top plan view of the crucible shown in FIG. 2; and

FIG. 4 is a bottom plan view of the crucible shown in FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the figures, crucible 10 embodying the present invention is made from a pure carbon having a density of 1.8 grams per cubic centimeter and a specific resistance for a cross-sectional rod of 0.56 inch in diameter of 0.00042 ohms per inch. The material is commercially available and identified as Arco Speer carbon, Grade No. 7lOGL. The crucible includes a cylindrical side wall 12 having an open top 14 for introducing the specimen 11 (FIG. 1) and an integrally formed bottom 16 defining at the interiorof the crucible, floor 18.

Floor 18 is rounded while the junction 20 of the exterior of the side wall 12 with the bottom of the crucible is chamfered around its periphery to reduce the contact area of the bottom surface 22 of the crucible. A buttonlike cylindrical projection 24 extends downwardly from the axis of the crucible and is employed for centering the crucible on the lower electrode as described below.

The electrical contact area for the crucible bottom with the lower electrode is the horizontally extending surface 22 which, in the preferred embodiment, is an annular ring as best seen in FIG. 4. The top rim 13 (best seen in FIG. 3) of the crucible also defines an annular contact surface for engagement with the upper electrode. The concavely rounded bottom 18 and the chamfered lower edge 20 of the crucible reduces the electrical current cross section path through the bottom of the crucible significantly and as is well-known, reducing the cross-sectional area of a conductive material increases its resistance. Thus, the PR heating effect within the bottom of the crubile itself is greatly increased to directly heat the bottom and increase the efficiency of operation of the crucible over prior crucible designs. The configuration of the furnace electrodes and the tips therefor is disclosed in detail in a copending U.S. patent application entitled ELECTRODE CONSTRUCTION FOR RESISTANCE HEATING FURNACE, Ser. No. 484,453 filed concurrently herewith, assigned to the present assignee and incorporated herein by reference. A brief description, however, is presented here to more fully describe the operation of the improved crucible.

The crucible is supported on the pedestal 32 of a lower copper electrode 30 movable toward and away from upper electrode 40 to alternately clamp and release the crucible between the electrodes respectively. Pedestal 32 includes a tip 34 having a significantly annular mating surface 35 configurated to seat against surface 22 of the crucible. Tip 34 also includes a central aperture 36 for receiving the centering projection 24 of the crucible and a plurality of cleaning notches 37 extending from aperture 36 radially outwardly through the otherwise continuous annular surface 35. Tip 34 is made of tungsten or a pressed mixture of 75% tungsten and 25% copper.

The lower electrode includes an annular recess 38 formed downwardly therein for receiving the downwardly projecting end 42 of the upper electrode. Sealing means 43 comprising O-rings extend around the lower peripheral edge of portion 42 for sealably fitting within recess 38 when the lower electrode 30 is moved upwardly in a direction indicated by arrow A by the control shaft 39 attached thereto.

The upper electrode includes a cylindrical cruciblereceiving chamber 44 extending upwardly therein and communicating with a cylindrical passageway 45 for introducing a sample into the crucible as well as a carrier gas such as helium. Extending around the junction of

passageways

44 and 45 is a washer-shaped insert 46 of the same material as tip 34 and which seats against rim 13 of the crucible. Tip 46 includes radially extending slots 46 to permit specimen gas and carrier gas to be discharged from the crucible during fusion. A discharge port 47 extends from the bottom of electrode 40 upwardly and communicates with a discharge tube 48 for transporting the specimen gas to the analyzer associated with the furnace. It is noted that the open end of the bottom of port 47 does not contact the floor of recess 38 such that gases are permitted ,to discharge through coupling 48 during analysis. A power supply 50 provides alternating current for actuating the electrodes to pass current through the crucible when the electrodes are in the closed position thereby clamping the crucible between

inserts

34 and 46.

In the preferred embodiment, the crucible is integrally formed by machining a solid rod of the carbon material noted above and the cylindrical side wall has an outer diameter of 0.56 inch and an inner diameter of 0.48 inch. The rounded bottom 18 has a 0.6 inch radius of curvature and the chamfer angle is 30. The button-like projection 24 has a diameter of 0.18 inch while the outer diameter of annular surface 22 is 0.31 inch. The height of the crucible from the floor 22 to rim 13 is 0.625 inch while the button-like projection 24 extends downwardly approximately 0.055 inch. The thickness of the crucible bottom 16 at BB is 0.2 inch. This dimension is significantly reduced over existing crucibles by the smoothly curved concave floor 18 and chamfer 20 and accounts for the reduced electrical current cross-sectional area. These dimensions are approximate and subject to normal tolerances of about The area of annular contact surface 22 is about 0.005 square inches while the internal cross section of the bottom in area B-B is approximately the mean diameter of 0.4 inch multiplied by the width of 0.2 inch and 11' or about 0.25 square inches.

It is seen in FIG. 1 that the bottom of the projection 24 does not contact the tip 34 of the electrodes 30 but serves only a centering function with the sole contact area between the electrode and the crucible being the annular surface 22. Should the crucible tip 34 wear after considerable use and the contact and the annular surface 35 of the tip become enlarged, the contact area with each crucible (one is used for each analysis) remains the same by virtue of the chamfer which prevents the contact area from increasing. It is noted that in some embodiments, the centering stud 24 may not be necessary and the crucible could be flat-bottomed. In such embodiments, however, the chamfered edge 20 is required to. reduce the bottom electrical current cross section area. These and other modifications to the preferred embodiment will, however, come within the spirit and scope of the present invention as defined by the appended claims.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows.

1. A resistance heating crucible made of a conductive material for fusion of a specimen and comprising a cylindrical side wall having an open end defining the top of said crucible and an enclosed end defining the bottom of said crucible, wherein the interior of said bottom is concavely rounded and the junction of the exterior of said side wall with the bottom is chamfered to reduce the cross-sectional area of the electric current path through said bottom of said crucible.

2. The crucible as defined in claim 1 and further including a button-like projection extending downwardly from the center of said crucible bottom.

3. The crucible as defined in claim 2 wherein said junction is chamfered at an angle of 30.

4. The crucible as defined in claim 1 wherein the exterior of said bottom is configurated to define a horizontally extending annular contact area.

5. The crucible as defined in claim 1 wherein said interior of said bottom is rounded at about 0.6 inch radius of curvature and said cross-sectional area is a maximum of about 0.25 square inches.

6. The crucible as defined in claim 5 and further including a cylindrical projection extending downwardly from the center of said bottom about 0.05 inch and having a diameter of about 0.19 inch.

7. In combination with the crucible as defined in claim 2, an electrode for supporting said crucible including a substantially annular contact surface surrounding said projection and a recess into which said projection extends, said recess having a depth greater than the length of said projection.

8. A carbon crucible for use in holding and heating a solid specimen between conductive electrodes of a furnace, said crucible including an open top, a cylindrical side Wall and an enclosed bottom, wherein the improvement comprises: said bottom including a concavely rounded interior defining the floor of said crucible and the junction of said bottom and said cylindrical side wall significantly chamfered around the periphery of said crucible to reduce the surface area of said bottom.

9. The crucible as defined in claim 8 and further including a cylindrical button-like projection extending downwardly from the center of said bottom.

10. For use in a fusion furnace, a carbon crucible comprising: a cylindrical side wall open at one end and enclosed at an opposite end to define a crucible bottom wherein the interior of said bottom is generally concavely curved and the exterior of said bottom is circular and chamfered around the edge adjoining said side wall to reduce the cross-sectional area of the current flow path through said crucible bottom.

Claims

3. The crucible as defined in claim 2 wherein said junction is chamfered at an angle of 30*.