Abstract:
A resistance furnace provides an improved upper and lower electrode construction with significantly increased coolant flow. The lower electrode has a tip design that significantly lowers the electrode tip temperature during an analysis. The upper and lower electrodes also cooperate with an improved crucible design to significantly reduce the power required to fuse a specimen contained in the crucible. The furnace uniformly heats the floor and lower side walls of a crucible, which lowers the power requirement for specimen fusion and provides higher structural benefits to provide consistent analysis and manufacturing yields. The crucible has a cylindrical body and pedestal base with an annular smoothly curved concave indentation therebetween.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119(e) and the benefit of U.S. Provisional Application No. 61/661,595 entitled RESISTANCE ANALYTICAL FURNACE, filed on Jun. 19, 2012, by Joshua N. Wetzel, et al., the entire disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to analytical furnaces and particularly to an improved electrode structure of a resistance furnace and the combination of the electrode structure with a resistance crucible. 
     Analytical furnaces heat specimens, such as chips and pin samples, and the like, typically ranging in mass from about 1 mg to about 1 gram in a resistive crucible, typically made of graphite. Resistance furnaces employ such graphite crucibles which are clamped between two electrodes to pass an electrical current through the crucible, heating specimens to temperatures of 2500° C.-3000° C. or higher. The gaseous byproducts of fusing the specimen are then swept by an inert gas, such as helium, through the furnace system to an analyzer for the subsequent analysis of the specimen gases of interest using suitable detectors. Such an analyzer is represented by Model ONH836, which is commercially available from Leco Corporation of St. Joseph, Mich. 
     The heating of a specimen in existing graphite crucibles requires a significant amount of electrical power, in the neighborhood of 7500 watts. This, in turn, requires the electrodes coupled to the graphite crucible to be water cooled. Even with water cooling and the use of tungsten/copper alloy tipped electrodes, the electrodes tend to wear. The use of replaceable tips, such as disclosed in U.S. Pat. No. 4,419,754, although protecting the body of the electrode, still require frequent replacement since, even though water cooled, the electrode tips can be subjected to temperatures as high as 1050° C. during use. Also, existing crucible geometries, such as represented by U.S. Pat. Nos. 3,636,229, 3,899,627, and 4,328,386, when heated to temperatures approaching or exceeding 3000° C., tend to have hot spots instead of uniformly heated floors. Also, their side walls allow a specimen, which is heated to a bubbling state, to overflow the crucible and, in some cases, contaminate the upper electrode. 
     There exists a need, therefore, to improve the performance of existing resistance furnaces, including reducing power consumption, improving electrode lifetime, and providing a crucible which, when used with the electrodes, fuses a variety of specimen shapes efficiently without contaminating the upper electrode. 
     SUMMARY OF THE INVENTION 
     The furnace of this invention provides an improved electrode construction with coolant flow and an electrode tip design that significantly lowers the tip temperature during an analysis, thereby greatly increasing the life of the tipped electrodes resulting in increased cycles of use before requiring replacement. The electrodes also cooperate with an improved crucible design to significantly reduce the power required to fuse a specimen contained in the crucible. The furnace uniformly heats the floor and lower side walls of a crucible, which is designed to enable a lower power requirement for specimen fusion and higher structural benefits to improve consistent analysis and manufacturing yields. 
     An electrode assembly for a resistance furnace of this invention includes a generally cylindrical upper electrode including a central axially extending aperture for admission of a sample into a crucible held within the electrodes in an enlarged cylindrical furnace area. The upper electrode includes a generally funnel-shaped lower end for receiving the crucible-holding pedestal of a lower electrode. The lower electrode has a pedestal for supporting a crucible and is shaped to be raised into sealable engagement within the funnel-shaped lower end of the upper electrode. The lower electrode includes a generally truncated conical recess formed in the bottom of the pedestal and a cooling water inlet and a cooling water outlet communicating with the conical recess. A nozzle is positioned in the conical recess of the lower electrode and has a discharge end closely positioned with respect to the interior wall of the truncated conical recess. The nozzle is coupled to the water inlet for projecting cooling water toward the pedestal of the lower electrode. 
     When such electrodes are combined with the use of a graphite crucible having a cylindrical body with a pedestal base with an inwardly extending concave annular indentation between the body and the base, the resultant furnace operation is highly efficient. Also, the electrodes have a greatly improved lifecycle before requiring replacement. 
     These and other features, objects and advantages of the present invention will become apparent upon reading the following description thereof together with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a fragmentary perspective view of an analyzer including a furnace which includes the electrode assembly of the present invention; 
         FIG. 2  is an enlarged perspective view of the analyzer shown in  FIG. 1 , shown with the cabinet cover removed to expose the furnace area and sample drop mechanism; 
         FIG. 3  is an exploded perspective view of the electrode assembly used in the furnace; 
         FIG. 4  is a front elevational exploded view of the upper and lower electrodes in an open position and shown with a crucible positioned on the lower electrode; 
         FIG. 5  is an enlarged cross-sectional view of the structure shown in  FIG. 4 , taken along section lines V-V of  FIG. 4 ; 
         FIG. 6  is a front elevational view of the electrode assembly shown in a closed position; 
         FIG. 7  is an enlarged cross-sectional view, taken along section line VII-VII in  FIG. 6 , showing the components of the furnace including the positioning of a crucible therein; 
         FIG. 8  is a pictorial cross-sectional view of the structure shown in  FIG. 7 , illustrating in dotted lines the water flow path for cooling the upper and lower electrodes; 
         FIG. 9  is a pictorial cross-sectional view of the structure shown in  FIG. 8 , illustrating in dotted lines the inert gas flow path through the furnace; 
         FIG. 10  is a fragmentary perspective view illustrating the relationship of the crucible and the tip member of the upper electrode shown also in  FIGS. 7-9 ; 
         FIG. 11  is a perspective view of the crucible; 
         FIG. 12  is a top plan view of the crucible; 
         FIG. 13  is a bottom plan view of the crucible; 
         FIG. 14  is a side elevational view of the crucible; and 
         FIG. 15  is a vertical cross-sectional view of the crucible, taken along section lines XV-XV in  FIG. 12 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring initially to  FIGS. 1 and 2 , there is shown an oxygen/nitrogen/hydrogen analyzer  10 , such as a model OHN836 commercially available from Leco Corporation of St. Joseph, Mich., but modified to include the present invention. The analyzer includes a touch screen control (not shown) for the operation of the furnace and components of the analyzer and an inclined sample loading carousel  14  of the type described in pending patent application Ser. No. 13/402,192, entitled S AMPLE  L OADING  C AROUSEL , which was filed on Feb. 22, 2012. The analyzer also includes an electrode power cleaning brush assembly  16  together with a vacuum cleaning mechanism  18  of the type described in U.S. patent application Ser. No. 13/358,096 entitled V ACUUM  C LEANING  S TRUCTURE FOR  E LECTRODE  F URNACE , which was filed on Jan. 25, 2012. The disclosures of both of these applications are incorporated herein by reference. 
     As seen in  FIGS. 1 and 2 , the analyzer  10  includes a lower electrode assembly  20  which includes a pedestal as described below for receiving a crucible  100  ( FIGS. 1, 4, 5 and 7-15 ). The lower electrode assembly  20  can be raised in a direction indicated by arrow A in  FIGS. 2 and 5  to insert a crucible positioned on the lower electrode into the furnace area  60  in the upper electrode assembly  40  ( FIGS. 3-7 ). 
     As seen in  FIGS. 3-7 , the electrode structure for the furnace  60  comprises a lower electrode assembly  20  and an upper electrode assembly  40 , both of which are water cooled. The lower electrode assembly  20  includes a lower electrode  22  made of copper and having a generally disk-shaped center body  24  and an upwardly projecting pedestal  26 . The electrode  22  includes a downwardly extending annular section  25 . The bottom of electrode  22  includes an upwardly extending truncated conical recess  28  which terminates in a cylindrical upper end section  28 ′ ( FIGS. 5 and 7 ). The top wall of section  28 ′ is relatively thin (0.050-0.080 inch) to provide heat transfer from the alloy tip  36  threaded onto projection  21 . Recess  28  houses a water projecting nozzle  30  ( FIGS. 7-9 ) which is coupled to the cooling water inlet  32  ( FIG. 8 ) in the water jacket  34  ( FIGS. 6 and 7 ) surrounding the lower annular section  25  of electrode  22 . The discharge upper end of nozzle  30  is closely spaced (0.45 inches) from the top wall of section  28 ′. The enlarged flared lower end of truncated conical recess  28  of the lower electrode  22  provides a significant internal surface area and volume to allow a significant cooling water flow rate of from about 0.95 to 1.05 gallons per minute (gpm) to cool the lower electrode  20  during an analysis. 
     Lower electrode  22  includes an upwardly extending circular projection  21  ( FIG. 3 ) on the top center of pedestal  26 , which has external peripheral threads  29  ( FIG. 7 ) to receive a cup-shaped tungsten/copper alloy (10% Cu, 90% W) tip  36 , which is internally threaded to screw onto center circular projection  21  when assembled, as seen in  FIGS. 5 and 7-9 . The facing surfaces of tip  36  and projection  21  are machined flat (i.e., to a geometric tolerance of 0.002 inch) to provide maximum electrical contact between the tip  36  and the electrode  22 . The inside of the cup-shaped tip  36  includes a circular recess  38  in which a button-like projection  27  ( FIG. 3 ) on the tip-receiving projection  21  extends. The tip  36  includes a centered upwardly projecting button  37  which, as seen in  FIGS. 5 and 7-9 , extends into an indentation  124  ( FIGS. 13 and 15 ) in the bottom of the pedestal  114  of crucible  100 . The button  37  has a height less than the depth of the indentation  124  in the bottom of pedestal  114  to leave a gap “g” ( FIG. 9 ) between the upper surface of button  37  and the floor  125  of the recess in pedestal  114  of about 0.020 inches. The only contact between the tip  36  and the pedestal  114  of crucible  100  is the annular area  39  ( FIGS. 3 and 5 ) of tip  36  surrounding button  37 , and the annular bottom surface  132  ( FIGS. 13-15 ) of the crucible. 
     O-ring seals  31  seal the electrode  22  within water cooling jacket  34 , as illustrated in  FIGS. 7-9 , while O-rings  33  seal the pedestal  26  within the upper electrode assembly  40  when in a closed position, as also illustrated in these figures. The water jacket surrounding the lower electrode  22  includes a water inlet  32  coupled to nozzle  30 , as seen in  FIG. 8 . The nozzle  30  includes a discharge opening  30 ′ for directing cooling water, as shown by arrow “W” in  FIG. 8 , against the inside hollow undersurface of electrode  22 . The water jacket  34  also includes an outlet  35  for discharging water from jacket  34 . The flow path of the water which cools the inside of the volume of recess  28  of the lower electrode near the area of the tip  36  and projection  21  is shown in dotted lines in  FIG. 8 . The water can be recirculated through a cooler in a conventional manner. 
     The lower electrode assembly  20  sealably extends within the upper electrode assembly  40 , as illustrated in  FIGS. 7-9 . For such purpose, the upper electrode assembly  40  likewise includes an electrode  42  surrounded by a cooling jacket  44  for receiving cooling water from a water inlet  46  ( FIG. 8 ). Cooling water flows around spiral fins  48  extending radially outwardly from electrode  42  and is discharged through outlet  51 . Upper and lower O-ring seals  53  seal water jacket  44  to the upper electrode  42 , as seen in  FIGS. 5 and 7-9 .  FIGS. 8 and 9  illustrate the preferred embodiment of the upper electrode fin construction, which provides enlarged water cooling channels  56  to significantly increase the flow rate of cooling water over the smaller channels shown in  FIGS. 3, 5, and 7 . The upper electrode water flow path is also illustrated by the dotted lines in  FIG. 8 . 
     The upper electrode  42  includes an upper funnel-shaped opening  41  for receiving samples from a sample drop mechanism  62  ( FIG. 2 ), which is mounted to the furnace  60  directly above the upper electrode  42 . Mechanism  62  is described in U.S. Pat. No. 6,291,802, the disclosure of which is incorporated herein by reference. The upper electrode  42  includes an axially extending cylindrical aperture or tube  43  communicating with opening  41  and extending downwardly and terminating in an enlarged cylindrical opening  45  defining a furnace chamber  60  which receives the crucible  100  and lower electrode  22 , as seen in  FIGS. 7-9 . The lower end of cylindrical opening  45  is beveled, enlarged, and outwardly tapered at wall  47  and has a cylindrical opening  49  for sealably receiving the lower electrode, as seen in  FIGS. 7-9 . The interface between tube  43  and the enlarged cylindrical opening  45  includes an annular tungsten carbide insert  50 , which engages the annular upper rim  115  of crucible  100 , as seen in  FIGS. 8-10 . The enlarged cylindrical opening  45  defines the furnace area  60 , which receives the graphite crucible  100  during the fusion process when electrical power is applied to the electrodes  22  and  40 . In order for the gaseous byproducts of fusion of a sample to escape the crucible  100 , radially extending slots  52  ( FIG. 10 ) are provided in the insert  50 . This provides a gas flow path, as illustrated in dotted lines in  FIG. 9 , for the helium which is injected through the tube  43  to gather the analyte gases from the crucible during fusion of a sample and exit through outlet  54  of the electrode  42  to the analyzer detectors for detecting analytes of interest. 
     This improved electrode construction provides significantly better water cooling, such that the upper and lower electrodes operate at cooler temperatures, averaging from 500° C. to 600° C. as compared to the prior electrodes which operated at a temperature range of from about 800° C. to 900° C. with hot spots exceeding 1000° C. This results in a significantly longer electrode life as well as the life of the electrode tip  36  and insert  50 . The improved cooling is partially responsible for this success as is the cooperation and synergistic relationship of the electrodes and tip with the design of crucible  100 , which allows fusing of a specimen with lower power consumption. The crucible  100  is described in detail in conjunction with  FIGS. 11-15 . 
     Referring initially to  FIGS. 11 and 14 , there is shown a crucible  100  which cooperates with the furnace electrodes to provide the greatly improved performance of the present invention. Crucible  100  is made of commercial grade graphite and is machined from a solid graphite rod. The crucible has a cylindrical body  112 , supported by a pedestal base  114  between which there is an annular inwardly extending concave indentation  116 . The crucible has an open mouth  118  at the top for receiving specimens ranging from about 0.1 gram to 1 gram in size. The specimens can be in any form but typically are pins or chips which likewise are frequently encased in a nickel basket serving as an accelerator during the fusion process. The sample can be introduced manually in some systems or preferably by automation through tube  43  in the upper electrode assembly  40  and a sample drop mechanism as described in U.S. Pat. No. 6,291,802. 
     The cylindrical body  112  includes an outer cylindrical wall  111  and an inner cylindrical wall  113 . As seen in  FIG. 15 , the interior of the crucible also includes a floor  120  which is concavely curved at a first radius of curvature R 1 . The floor communicates with the interior side wall  113  of the cylindrical body at a curved junction  122  having a second radius of curvature R 2 . As seen in  FIGS. 13 and 15 , the pedestal base has a centrally located circular indentation  124 . The annular indentation  116  includes an upper wall  126  and a lower wall  128 , which converge at an apex  130  having a radius of curvature R 3 . The smoothly curved, inwardly concave annular indentation  116  has the walls  126 ,  128  diverging outwardly from apex  130  at an angle α of 56° to 60°, as illustrated in  FIG. 15 . The angle β from the lower wall  28  to the floor is from 22°-24°. 
     The geometry of crucible  100 , as best seen in  FIG. 15 , including the arcuate indentation  116  and circular indentation  124 , results in a current choke zone circled and identified by the letter C in  FIG. 15 . With this design, the current flows from the annular bottom surface  132  of the pedestal, which engages the annular surface  39  of the tip  36  on projection  21  of lower electrode  22  of the furnace, as described above, through the choke zone C. The upper annular surface  115  of the crucible  100  engages the insert  50  of upper electrode  42  of the furnace, such that the current passes through the crucible between surfaces  132  and  115 . The geometry of the crucible with the dimensions noted below resulted in a uniform heating of the floor  120  of the crucible and the side wall  113  through the zone between the floor  120  up to the area of arrow A in  FIG. 15 . Unexpectedly, this geometry allowed heating of a specimen positioned in a crucible at a much lower power than is typical. 
     As noted above, in prior furnaces, the furnace power was approximately 7500 watts when a voltage of about 6.25 volts was supplied at a current of 1200 amps. With the present crucible  100 , at the same 6.25 volts, only 800 amps of current is required providing a power of 5000 watts to achieve the same about 3000° C. fusion temperature within the crucible  100 . This unexpected power savings of approximately 30% not only saves the laboratory using the furnace for analysis of specimens electricity costs but also allows the manufacture of the furnace to employ less expensive components since a lower current power supply can be used. The dimensions identified below and shown in  FIGS. 12, 13, and 15  provided optimized performance for the preferred embodiment of the invention. Although these dimensions were optimized for the crucible, they may be varied somewhat. The depth, diverging angle, and radius of curvature of indentation  116 , however, is of great importance. 
     The diameter D 3  ( FIG. 13 ) of the indentation  124  in the pedestal was about 0.19 inches, with a depth H 2  ( FIG. 15 ) of about 0.056 inches. The overall crucible height H 1  ( FIG. 15 ) is 1.1 inches, while the height of pedestal base  114  H 3  ( FIG. 15 ) was about 0.060 inches. The distance H 4  between the upper edge of the pedestal base  114  and the lower edge of the outer cylindrical body  112  of the crucible (i.e., the open mouth of the arcuate indentation  116 ) was about 0.224 inches. The radius of curvature R 1  of floor  120  was 0.35 inches while the junction of the floor  120  to the side wall  113  has a radius of curvature R 2  of 0.060 inches. The indentation  116  has diverging upper and lower side walls  126 ,  128  at an angle α of from about 56° to about 60°, and preferably about 58°. The angle β from the lower wall  28  to the floor is from 22°-24°. The radius of curvature R 3  of the smoothly curved annular indentation  116  is about 0.050 inches. The depth L 1  ( FIG. 15 ) of indentation  116  is about 0.166 inches. The outer diameter D 1  ( FIG. 12 ) of the crucible was 0.50 inches while the inner diameter D 2  was 0.39 inches, leaving a wall thickness of 0.055 inches. 
     It is expected that the specific dimensions given can vary within the normal manufacturing tolerances of machining of such crucibles and may be varied by as much as up to 3%, although it was discovered that the smoothly curved annular indentation  116  dimensions and geometry are critical to the successful functioning of the crucible to provide the unexpected uniform heating as well as lower power consumption. The crucible is subjected to approximately 83 pounds force when positioned between the upper and lower electrodes, as disclosed above. The maximum resistance area at the lower part of crucible  100  is the current choke area identified by arrow C in  FIG. 15  and results in, together with the indentation  116 , the uniform heating of the floor  120  of the crucible and the side walls up to the area of arrow B ( FIG. 15 ). This provides improved uniform heating of specimens introduced into the crucible and prevents the fused specimen from bubbling over the top of the crucible through mouth  118  contaminating the upper electrode. The crucible height H 1  is slightly (10%) higher than the usual crucible construction, which also assists in the prevention of specimens reaching the insert  50  of the upper electrode  42 . 
     It will become apparent to those skilled in the art that various modifications to the preferred embodiment of the invention as described herein can be made without departing from the spirit or scope of the invention as defined by the appended claims.