Integrally heated electrochemical cell method and apparatus

• US 4,659,435 A
• Filed: 08/09/1985
• Issued: 04/21/1987
• Est. Priority Date: 02/18/1983
• Status: Expired due to Term

• Current Assignee: United Process Control Inc. (Micronics Filtration Holdings, Inc.)
• Original Assignee: Corning Glass Works (Corning Incorporated)
• Inventors: Kane, William T., Brothers, Jack A., Brouneus, Harold A., Layton, Margaret M., Walsh, Paul L.
• Primary Examiner(s): Kaplan, G. L.
• Application Number:
  US06/764,234
• Time in Patent Office: 620 Days
• Field of Search: 204/406, 204/408, 204/412, 204/424, 204/425, 204/426, 204/427, 204/428, 204/429, 204/274, 204/262, 204/1 S
• US Class Current: 205/785
• CPC Class Codes: G01N 27/4067 Means for heating or controlling the temperature of the solid electrolyte
• CPC Class Codes: G05D 23/1913 delivering a series of pulses
• CPC Class Codes: G05D 23/20 with sensing elements having variation of electric or magnetic properties with change of temperature G05D23/13 takes precedence

Abstract

A novel electrochemical cell including a solid electrolyte heated to an elevated temperature for operation and an integral cell electrode/heater for heating the electrolyte. Solid electrolyte embodiments are described. The integral electrode/heater is applied to a surface of the solid electrolyte and is resistively heated by an alternating electric current. A portion of the solid electrolyte may also be resistively heated. The described or comparable cells may be incorporated into an oxygen detector or similar apparatus where the apparatus is operated by alternately heating the cell and measuring the emf developed by the cell across its electrode.

The invention is also, in part, a unique, solid electrolyte-integral cell electrode/heater configuration which provides a zone of uniform maximum heating at a predetermined location within the electrolyte and which, when used with a suitable resistive heating electric current, confines the current to the integral electrode/heater prolonging cell life. Another aspect of the invention is the use of a radio frequency alternating electric current for resistively heating an electrochemical cell. Preferably, the radio frequency selected is sufficiently high so as to eliminate any offsets in the emf developed by the cell which are caused by the heater current. The invention also includes an apparatus for measuring the concentration of particular gases, such as oxygen, incorporating either and, preferably, both other inventive aspects of the invention. The preferred cell configuration reduces the complexity of such an apparatus by eliminating the auxiliary heat source and provides a more accurate and reliable electrochemical sensing cell. Radio frequency heating allows the operations of cell heating and emf measurement to proceed independent and concurrently and provides a continuously responding, self-heating detection apparatus.

Claims

1. An electrochemical cell with integral heater comprising:

a solid electrolyte having at least a hollow tubular portion with an outer tubular surface and opposing inner tubular surface; and

an integral cell electrode/heater layer covering substantially all but a pair of opposing, substantially parallel, longitudinally extending strips of the outer tubular surface.

2. The electrochemical cell of claim 1 wherein said electrolyte tubular portion has a substantially circular cross-section with a substantially uniform cross-sectional thickness and said layer has a substantially uniform composition and thickness along at least a major proportion of the outer tubular surface.

3. The electrochemical cell of claim 1 wherein said solid electrolyte further comprises a hollow, substantially hemispherical portion integrally formed with one end of the tubular portion, the hemispherical portion having a convex outer surface and an opposing concave inner surface and wherein the layer covers at least a major proportion of the convex outer surface.

4. The electrochemical cell of claim 3 wherein said strips extend from an end of said layer proximal a remaining open end of the tubular portion of the electrolyte to the hemispherical portion of the electrolyte and wherein said integral layer covers the entire convex outer surface.

5. The electrochemical cell of claim 3 wherein the opposing parallel strips of exposed electrolyte extend from an end of the layer proximal a remaining open end of the tubular portion of the electrolyte and divide said layer into two portions, the two layer portions beginning at the end of the layer proximal the remaining open end of the tubular portion of the electrolyte and joining at the hemispherical portion end of the tubular electrolyte portion to form a single continuous layer, and said layer having a maximum total resistance of about one-half ohm or less at room temperature.

6. The electrochemical cell of claim 1 further comprising a second electrode layer contacting said inner tubular surface of the electrolyte opposite said integral cell electrode/heater layer and extending along at least a portion of the length of the tubular portion exposed by the two opposing strips.

7. The electrochemical cell of claim 6 wherein the two electrode layers are each formed from a material comprising a major proportion by weight of platinum and are porous to gaseous oxygen;

and wherein said solid electrolyte is formed from a material comprising a major proportion by weight of zirconia.

8. An electrochemical apparatus comprising:

an electrochemical cell including an electrolyte, a first integral cell electrode and a second cell electrode means and adapted for developing an emf between the first integral cell electrode and the second cell electrode during operation of the cell;

generation means for supplying radio frequency alternating current; and

circuit means including a pair of leads each having an end coupled with a different part of said first integral cell electrode and an opposing end coupled with said generation means connecting said generation means in a circuit through said cell across said first integral cell electrode for resistively heating at least said first integral cell electrode portion of the cell with said radio frequency alternating electric current.

9. The electrochemical apparatus of claim 8 wherein said alternating electric current has a frequency of alternation sufficiently high that the emf developed by the electrochemical cell is substantially unaffected by any further increase of the frequency of alternation of said alternating current.

10. The electrochemical apparatus of claim 8 wherein the frequency of the alternating current is between about 3,000 and 300,000 hertz.

11. The electrochemical apparatus of claim 10 wherein the frequency of the alternating current is further between about 30,000 and 100,000 hertz.

12. The electrochemical apparatus of claim 8 further comprising:

temperature sensing means for generating a signal related to the temperature of the electrochemical cell; and

control means responsive to the temperature signal for controlling the power level of the alternating current supplied by said generation means through said cell.

13. The electrochemical apparatus of claim 12 wherein said control means comprises means for automatically switching the alternating current on and off.

14. The electrochemical apparatus of claim 12 wherein said control means comprises:

a radio frequency oscillator; and

means for regularly activating said radio frequency oscillator for varying periods of time.

15. The apparatus of of claim 14 wherein said means for regularly activating comprises:

amplifier means responsive to said temperature signal for outputting a signal related to the difference between a temperature indicated by said temperature signal and a predetermined temperature; and

pulse width modulated oscillator means responsive to said amplifier means output signal for outputting a signal of varying duration at regular time intervals to said radio frequency oscillator.

16. The electrochemical apparatus of claim 15 wherein said radio frequency oscillator outputs a signal of no more than one polarity and further comprising current generator means connected across said pair of leads and responsive to said radio frequency oscillator output signal for generating an alternating current circuiting through said pair of leads with a frequency of alternation equal to the radio frequency of said reference oscillator output signal.

17. The apparatus of claim 8 wherein:

said electrolyte is solid and includes a hollow, tubular portion with an outer tubular surface and an opposing inner tubular surface and an open tubular end; and

said first integral cell electrode comprises;

an electrically conductive layer covering a major proportion of the outer tubular surface of the electrolyte; and

the cell further comprises;

a pair of strips of exposed electrolyte outer tubular surface extending along at least part of the tubular portion of the electrolyte, the strips dividing the electrode layer into two portions from an end of the layer proximal the open tubular end of the electrolyte, the electrode layer being circumferentially continuous about the electrode outer surface at an opposing end of the electrolyte; and

each lead extends from an end of a different one of the two portions of the electrode layer to the generation means whereby substantially all of the electrode layer is connected in a circuit with the generation means.

18. The electrochemical apparatus of claim 17 where the resistance of the electrode layer between the two lead contact points is about one-half ohm or less at room temperature.

19. An electrochemical apparatus responding to a predetermined gaseous substance in a sample gas comprising:

an electrochemical cell, the cell comprising;

a solid electrolyte exhibiting an increased conductivity to ions of the predetermined gaseous substance when heated,

first electrode means contacting the surface of the solid electrolyte at a first location for forming a gas electrode with the sample gas and the electrolyte and

second electrode means contacting the surface of the solid electrolyte at a second location for generating a known, predetermined potential with said solid electrolyte,

the electrochemical cell developing an emf between the two electrode means related to the concentration of the predetermined gaseous substance in the sample gas at the first electrode means; and

electric current source means connected in an electrical circuit across only an electrode means portion of the cell and adapted for passing a radio frequency electric alternating current through only said electrode means portion of the cell.

20. The electrochemical apparatus of claim 19 further comprising a pair of electrically conductive leads extending from different portions of one of said first and second electrode means to the electric current source means.

21. The electrochemical apparatus of claim 20 wherein the solid electrolyte includes a hollow tubular portion with an outer tube surface and an opposing inner tube surface;

wherein the one electrode means includes an electrically conductive layer porous to the gaseous substance and covering substantially all but a pair of opposing longitudinally extending strips of the outer tube surface dividing the layer into two sections extending along the tube portion of the electrolyte; and

wherein each of said leads extends from a different one of said two sections.

22. The electrochemical apparatus of claim 21 wherein said tubular portion is substantially circular in cross-section with a substantially uniform cross-sectional thickness and said layer has a substantially uniform composition and thickness along said outer tube surface.

23. The electrochemical apparatus of claim 21 wherein said electrolyte further includes a hollow, substantially hemispherical portion integrally closing one end of the tubular portion and having an outer convex surface and an opposing inner concave surface;

and wherein the layer is circumferentially continuous at the outer convex surface.

24. The electrochemical apparatus of claim 23 wherein said opposing longitudinally extending strips are substantially parallel, divide the layer into sections of substantially equal size and extend from an end of said layer proximal an opposing open end of the tube portion to said hemispherical portion of said solid electrolyte;

and wherein said layer covers the entire outer convex surface.

25. The electrochemical apparatus of claim 21 wherein said one electrode means further has a reduced resistivity in the vicinity of each of the contacting conductive leads.

26. The electrochemical apparatus of claim 25 wherein said one electrode means further includes a pair of metal foil pads each pad being located between the electrode layer and an end of a different one of the two conductive leads.

27. The electrochemical apparatus of claim 21 wherein the remaining electrode means comprises a second, gas porous, electrically conductive layer circumferentially covering a portion of the inner tube surface over an area extending along at least a portion of the tube surface exposed by the opposing strips.

28. The electrochemical apparatus of claim 27 wherein the two electrode layers are each formed from a material comprising a major proportion by weight of platinum and said solid electrolyte is formed from a material comprising a major proportion by weight of zironia.

29. An improved method of heating an electrochemical cell formed by an electrolyte and a pair of electrode means separately contacting the electrolyte for developing an emf therebetween comprising the steps of:

generating an alternating electric current having a radio frequency level of alternation; and

passing the alternating electric current through at least one of said pair of electrode means while passing no more than a negligible portion of said alternating electric current through said electrolyte.

30. The improved method of claim 29 wherein the generating step further comprises the step of:

generating an alternating current with a frequency of alternation sufficiently high as to eliminate any offset in the manitude of the emf developed by the electrochemical cell caused by the frequency of the alternating current.

31. The improved method of claim 29 wherein the passing step further comprises the step of passing the radio frequency alternating current through only one of the two cell electrode means.

32. The improved method of claim 29 wherein said generating step further comprises generating an alternating electric current with a frequency of alternation of between about 30,000 and 100,000 hertz.

33. The improved method of claim 29 further comprising the step of sensing the temperature of the cell and wherein said step of generating further comprises varying the power of the alternating electric current passed through the cell in response to said sensing step.

34. An improved method of heating an electrochemical cell, the cell comprising an electrolyte and a pair of cell electrodes contacting the electrolyte, the method comprising the step of:

resistively heating at least one of the two cell electrodes while no more than negligibly resistively heating the electrolyte.

35. The improved method of claim 34 wherein said resistive heating step further comprises the steps of:

generating an electric current; and

passing said electric current through at least one cell electrode means while passing no more than a negligibl portion of said electric current through the electrolyte.

36. The improved method of claim 35 wherein said generating step further comprises the step of generating an alternating electric current having a frequency of between about 3,000 and 300,000 hertz.

37. The improved method of claim 34 wherein the electrolyte is a solid, wherein each of said two cell electrode means contacts one of a pair of opposing surfaces of he electrolyte and wherein the resistive heating step further comprises the steps of:

warming a zone of the electrolyte to a maximum temperature with said resistively heated one electrode means, the zone extending through the electrolyte between the two electrode means and covering an area of at least several square centimeters; and

thereafter maintaining the temperature of the electrolyte in the zone within about 10°

C. or less of said maximum temperature by resistively heating said one electrode means.

38. The improved method of claim 34 wherein the electrolyte is a solid and at least one of the electrodes further comprises an electrically conductive layer covering at least a portion of an outer surface of the solid electrolyte and the resistive heating step further comprises the step of:

passing a substantially uniform electric current power density through a major proportion of the electrically conductive layer.

39. An improved method of operating a device for generating a signal related to a concentration of a particular gas in a sample gas, said device comprising an electrochemical cell having an electrolyte heated to an elevated temperature for operation, a first electrode means exposed to the sample gas and a second electrode means developing an emf with the first electrode means, sample gas and electrochemical cell having a magnitude related to the concentration of the particular gas in the sample gas, comprising the steps of:

generating radio frequency alternating current; and

passing said alternating electric current through at least a portion of one of the electrode means contacting the electrolyte for resistively heating only the one electrode means portion of the cell with said current; and

generating an emf with the cell during said current generating and passing steps having a magnitude releated to the concentration of the particular gas in the sample gas.

40. The improved method of claim 39 wherein said step of generating further comprises a step of:

generating an alternating current having a frequency of alternation sufficiently high that the magnitude of the emf developed by the electrochemical cell is substantially uneffected by further increase in said frequency of said alternating electric current.

41. The improved method of claim 40 wherein said electrolytic cell includes a zirconia electrolyte, said particular gas is oxygen and said electrolyte is warmed to a temperature of at least about 600°

C.

42. The improved method of claim 40 further comprising during said passing step the step of:

simultaneously generating a signal related to the magnitude of the emf being developed by the cell for indicating the concentration of the particular gas in the sample gas.

43. The improved method of claim 42 further comprising during the current generating and passing steps the step of simultaneously sensing the temperature of the electrochemical cell;

and said generating step further comprises the step of varying the power of the alternating electric current in response to said temperature sensing step.

44. The improved method of claim 43 wherein said electrolyte is solid and said one electrode means comprises a solid, electrically conductive layer contacting at least a portion of a surface of the solid electrolyte and said passing step further comprises the step of:

passing said alternating electric current through the layer while passing no more than a negligible portion of the alternating electric current through said solid electrolyte.

45. The improved method claim 42 wherein said electrolytic cell includes a zirconia electrolyte, said particular gas is oxygen and said electrolyte is warmed to a temperature of at least about 600°

C.

46. The improved method of claim 39 wherein said electrolyte is solid and said one electrode means includes a solid, electrically conductive layer contacting at least a portion of a surface of the solid electrolyte and said passing step further comprises the step of:

passing said alternating electric current through the layer while passing no more than a negligible portion of the alternating electric current through said solid electrolyte.

47. An electrochemical cell comprising:

an electrolyte;

integral cell electrode/heater means contacting the electrolyte and adapted for heating at least a portion of the electrolyte to an elevated temperature for ionic conduction operation; and

a pair of lead means each extending from a different location on the integral cell electrode/heater means for coupling said integral cell electrode/heater means in a circuit across an electric current source; and

a second cell electrode means separately contacting the electrolyte and adapted for developing an ionic conduction across the heated electrolyte and a related emf with the integral cell electrode/heater.

48. The cell of claim 47 wherein said electrolyte is a solid at said elevated temperature and has a surface and said integral cell electrode/heater means comprises an electrically conductive layer contacting at least a portion of the surface of the solid electrolyte.

49. The cell of claim 48 wherein the electrolyte has a hollow tubular form, an outer surface and an opposing inner surface and said layer covers a major proportion of said outer surface.

50. The cell of claim 49 wherein said hollow tubular form of said electrolyte has an open end and a closed end and wherein said layer includes a circumferentially continuous portion at the closed end and a pair of separate legs extending from the circumferentially continuous portion to an end of said layer proximal said open end of the electrolyte.

51. The cell of claim 50 wherein said integral cell electrode/heater means further comprises a pair of metal foil pads each attached to a separate one of the pair of legs of the layer so as to effectively reduce the surface resistivity of the integral cell electrode/heater means at said pads.

52. The cell of claim 49 wherein said layer covers all of said hollow tubular form of the electrolyte but a pair of stripes of exposed electrolyte surface, extending along the hollow tubular form and dividing part of said layer extending along the hollow tubular form into a pair of legs.

53. The cell of claim 48 wherein the layer is gas porous.

54. The cell of claim 48 wherein said layer is formed from a material comprising platinum.

55. The cell of claim 54 wherein a chemical bond is formed between at least a portion of said layer and said solid electrolyte.

56. The cell of claim 47 in a combination further comprising:

electric current source means electrically connected in a circuit across said pair of lead means and circuiting an alternating electric current across said integral cell electrode/heater means through said pair of leads means.

57. The cell of claim 47 in a combination further comprising:

electric current source means electrically connected in a circuit across the integral cell electrode/heater means for circuiting an electric current through at least a portion of the integral cell electrode/heater means.

58. The combination of claim 57 wherein said electric current source means comprises:

switch means for controlling the electric current circuited through the integral cell electrode/heater means.

59. The combination of claim 57 wherein said electrolyte is a solid at said elevated temperature and has a surface and said integral cell electrode/heater means comprises an electrically conductive layer contacting a portion of the surface of the solid electrolyte.

69. The electrochemical apparatus of claim 57 wherein said identified compound is oxygen and said electrolyte is formed from a material comprising stabilized zirconia.

70. The electrochemical apparatus of claim 57 wherein the electric current source means circuits an alternating electric current through at least a portion of the one electrode means.

60. An electrochemical apparatus responding to a predetermined gaseous substance in a sample gas comprising:

an electrochemical cell, the cell comprising;

a solid electrolyte exhibiting a conductivity to ions of the predetermined gaseous substance when heated above room temperature,

first electrode means contacting the surface of the solid electrolyte at a first location for forming a gas electrode with said sample gas and the electrolyte, and

second electrode means contacting a surface of the solid electrode at a second location for forming a reference electrode generating a known, predetermined ionic potential with said solid electrolyte; and

an electric current source means electrically connected across one of the two electrode means for resistively heating at least a portion of the one electrode means sufficiently for the cell to develop an ionic emf between the two electrode means related to the concentration of the predetermined gaseous substance in the sample gas.

61. The electrochemical apparatus of claim 60 wherein said one electrode means comprises an electrically conductive layer contacting the solid electrolyte.

62. The electrochemical apparatus of claim 61 wherein the layer is chemically bonded to the solid electrolyte.

63. The electrochemical apparatus of claim 61 wherein said electrolyte is a material comprising a major proportion by weight of zirconia and said layer is a material comprising a major proportion by weight of platinum and a minor proportion by weight of a substance chemically bonding to the zirconia.

64. The electrochemical apparatus of claim 61 wherein the solid electrolyte has a hollow tubular form with an outer surface and an opposing inner surface and said layer covers a major proportion of said outer layer.

65. The electrochemical apparatus of claim 64 wherein the one electrode means layer is gas porous and the remaining electrode means comprises a second gas porous, electrically conductive layer contacting the inner surface opposite the one electrode layer.

66. The electrochemical apparatus of claim 64 wherein said layer is circumferentially continuous at one end of the tubular portion of the electrolyte and divided into a pair of separate, substantially equal legs along the remainder of the tubular portion of the electrolyte by a pair of exposed strips of electrolyte outer tubular surface;

and further comprising;

a pair of leads each connecting a separate one of the pair of legs with said electric current source means.

67. The electrochemical apparatus of claim 66 wherein said remaining electrode means comprises a second gas porous, electrically conductive layer contacting the inner tubular surface opposite at least a portion of said outer layer between said one end of the electrolyte and said leads contacting the outer layer.

68. The electrochemical apparatus of claim 67 wherein said electric current source means comprises a step down transformer connected by said leads across said layer.

71. The electrochemical apparatus of claim 60 further comprising:

a pair of leads extending from different parts of the one electrode means across said electric current source means.

72. A method of heating an electrolyte of an electrochemical cell to an elevated cell ionic conduction operating temperature, the electrochemical cell being formed by the electrolyte, a first cell electrode and a second cell electrode, the method comprising the step of:

circuiting an alternating electric current through only a cell electrode portion of the cell.

73. The method of claim 72 wherein said alternating electric current circuited through said cell has a 60 hertz frequency of alternation.

74. A method of heating an electrolyte of an electrochemical cell to an elevated cell ionic conduction operating temperature, the electrochemical cell being formed by the electrolyte, a first cell electrode and a second cell electrode, the method comprising the step of:

resistively heating at least a portion of one of the two cell electrodes contacting the electrolyte to at least said elevated cell ionic conduction operating temperature.

75. The method of claim 74 wherein the resistive heating step comprises the step of circuiting an alternating electric current through at least a portion of only the one of the two cell electrodes.

76. The method of claim 75 further comprising a second subsequent resistive heating step of passing an alternating electric current through at least said portion of the one cell electrode and through at least part of the solid electrolyte.

77. The method of claim 74 wherein said electrolyte is a solid and ionically conductive at said operating temperature and said method further comprises the step of:

resistively heating at least a portion of the solid electrolyte and said portion of the one electrode after said cell has been heated by said one electrode.

78. A method of operating an apparatus for sensing a particular gaseous substance, the apparatus comprising an electrochemical cell heated to an elevated temperature for ionic conduction operation and circuit means for responding to an ionic emf developed by the electrochemical cell, the electrochemical cell comprising a solid electrolyte exhibiting increased conductivity to ions of the gaseous substance when heated to the elevated temperature, a first cell electrode contacting the electrolyte and exposed to a sample gas and a second cell electrode also contacting the electrolyte at a separate location and generating a predetermined potential, the electrochemical cell developing an ionic emf between the two cell electrodes related to the concentration of the particular gaseous substance in the sample gas, comprising the steps of:

resistively heating at least a portion of one of the two electrodes to said elevated temperature; and

measuring the ionic emf developed between the two cell electrodes after the electrolyte has been heated to said elevated temperature.

79. The method of claim 78 further comprising the step of terminating the resistive heating step before the measuring step.

80. The method of claim 79 wherein the one electrode is again resistively heated after said measuring step and before a second, similar measuring step.

81. The method of claim 78 further comprising during the step of resistively heating at least a portion of the one electrode, the step of simultaneously resistively heating a portion of the solid electrolyte.

82. The method of claim 81 further comprising the steps of terminating the two resistive heating steps before the measuring step.

83. The method of claim 78 wherein the resistive heating step comprises the step of circuiting an alternating electric current through only one of the two cell electrodes.

84. The method of claim 83 further comprising the step of circuiting a portion of the alternating electric current through a portion of the solid electrolyte.

85. The method of claim 83 further comprising the steps of:

measuring the temperature of the cell; and

terminating the alternating electric current in response to the temperature measuring step.

86. The method of claim 85 further comprising the step of initiating the ionic emf measuring step in response to said terminating step.

87. A method of operating an apparatus for sensing a particular gaseous substance, the apparatus comprising an electrochemical cell heated to an elevated temperature for ionic conduction operation and circuit means for responding to an ionic emf developed by the electrochemical cell, the electrochemical cell comprising a solid electrolyte exhibiting increased conductivity to ions of the gaseous substance when heated to the elevated temperature, a first cell electrode contacting the electrolyte and exposed to a sample gas and a second cell electrode also contacting the electrolyte at a separate location and generating a predetermined potential, the electrochemical cell developing an ionic emf between the two cell electrodes related to the concentration of the particular gaseous substance in the sample gas, comprising the steps of:

circuiting an alternating electric current through only a cell electrode portion of the cell sufficient to heat said cell; and

measuring the emf developed between the two cell electrodes after the electrolyte has been heated to said elevated temperature.

88. An improved method of operating a device for generating a signal related to the concentration of a particular gas in a sample gas, said device comprising an electrochemical cell heated for ionic conduction operation, exposed to the sample gas and developing an ionic emf having a magnitude related to the concentration of the particular gas in the sample gas comprising the steps of:

generating an alternating electric current in a substantially constant voltage band;

passing said alternating current through at least a portion of the electrochemical cell for resistively heating the cell; and

cyclicly switching the alternating electric current off during said generating and passing steps for irregular portions of a series of consecutive, predetermined cycle intervals.

89. The improved method of claim 88 further comprising during said cyclicly switching step, the steps of:

sensing the ionic emf developed by the electrochemical cell; and

generating a signal related to the magnitude of the ionic emf being developed by the cell for indicating the concentration of the particular gas in the sample gas.

90. An electrochemical cell with integral heater comprising:

a solid electrolyte;

an integral cell electrode/heater means comprising an electrically conductive layer contacting and covering a portion of the solid electrolyte; and

an electric current source means electrically connected across only the electrode/heater means for resistively heating at least a portion of the electrode/heater means.