Solid electrode

1. A reactor comprising an anode, a cathode and at least one bipolar electrode having first and second major working surfaces positioned therebetween wherein the at least one bipolar electrode consists essentially of diamond and the diamond comprises a dopant such that the diamond is conductive and has an electrical resistivity of 1 MΩcm or less and wherein the average concentration of the dopant in a region of at least one of the major working surfaces to a depth of 50 nm is at least 8×10 19 atoms/cm 3 .

2. A reactor according to claim 1 , wherein the average concentration of the dopant in a region of both the major working surfaces to a depth of 50 nm is at least 8×10 19 atoms/cm 3 .

3. A method of treating a fluid, wherein the fluid to be treated is passed through a reactor as defined in claim 1 .

4. A method according to claim 3 , wherein the anode and the cathode are formed from the same material and the polarity of these electrodes is reversed periodically.

5. A method according to claim 3 , wherein the fluid to be treated is passed through the reactor in a serpentine manner.

6. A method according to claim 3 , wherein the fluid to be treated is passed through the reactor in a parallel manner.

7. A method according to claim 3 , wherein a salt is added to the fluid to be treated.

8. A method according to claim 3 , wherein the fluid is at a temperature of greater than 30° C.

9. A method according to claim 3 , wherein the flow through the reactor is at least 0.5 m 3 /hour.

10. A method according to claim 3 , wherein the flow through the reactor per bipolar electrode is at least 0.1 m 3 /hour.

11. A method according to claim 3 , wherein the fluid to be treated is waste water.

12. A method according to claim 11 , wherein a current density of at least 1000 A/m 2 is applied to the electrodes.

13. A method according to claim 3 , wherein the fluid to be treated is pharmaceutical effluent.

14. A method according to claim 1 , wherein a potential difference in the range from 0.5 to 20 V is applied between any anode and adjacent cathode.

15. A reactor comprising an anode, a cathode and at least one bipolar electrode having first and second major working surfaces positioned therebetween wherein the at least one bipolar electrode consists essentially of diamond and the diamond comprises a dopant such that the diamond is conductive and has an electrical resistivity of 1 MΩcm or less and wherein the average concentration of the dopant in a region of at least one of the major working surfaces to a depth of 50 nm is greater than ⅕ of the average concentration of the dopant in the remainder of the at least one bipolar electrode.

16. A reactor according to claim 15 , wherein the average concentration of the dopant in a region of both of the major working surfaces to a depth of 50 nm is greater than ⅕ of the average concentration of the dopant in the remainder of the bipolar electrode.

17. A reactor according to claim 15 , wherein the dopant is boron.

18. A reactor according to claim 17 , wherein the average concentration of boron is less than 3×10 21 atoms/cm 3 .

19. A reactor according to claim 15 , wherein the dopant is dispersed uniformly throughout the diamond bipolar electrode.

20. A reactor according to claim 19 , wherein the uniformity is such that the concentration of dopant atoms in any 1 mm 3 volume does not vary from the concentration of dopant atoms in any other 1 mm 3 by more than 50%.

21. A reactor according to claim 20 , wherein for a series of at least 5 measurements taken at regularly spaced intervals along a line perpendicular to a major working surface of the bipolar electrode, the maximum dopant concentration measured is less than 150% of the mean value and the minimum dopant concentration is more than 50% of the mean value.

22. A reactor according to claim 15 wherein the diamond is polycrystalline diamond.

23. A reactor according to claim 15 , wherein the at least one bipolar electrode has an electrical resistivity in the range from 0.005 Ωcm to 0.50 Ωcm.

24. A reactor according to claim 15 , wherein one of the major working surfaces is the as-grown nucleation face.

25. A reactor according to claim 15 , wherein one of the major working surfaces is the as-grown growth face.

26. A reactor according to claim 15 , wherein the first major working surface is the as-grown nucleation face and the second major working surface is the as-grown growth face.

27. A reactor according to claim 15 , wherein the at least one bipolar electrode is a planar disc.

28. A reactor according to claim 15 , wherein the at least one bipolar electrode has a thickness in the range from 0.2 mm to 2 mm.

29. A reactor according to claim 15 , wherein the at least one bipolar electrode has a major working surface having an area of at least 10 cm 2 .

30. A reactor according to claim 15 , wherein the at least one bipolar electrode has a thickness uniformity such that the minimum thickness as measured using the 17-point array technique is more than 60% of the mean thickness and the maximum thickness measured is less than 135% of the mean thickness.

31. A reactor according to claim 15 , wherein the at least one bipolar electrode has a longest dimension of at least 10 mm.

32. A reactor according to claim 15 , wherein the material comprising the at least one bipolar electrode has an absolute strength measured in three-point bend of greater than 300 MPa.

33. A reactor according to claim 15 which comprises at least 2 bipolar electrodes positioned between the anode and the cathode.

34. A reactor according to claim 15 which comprises at least 20 bipolar electrodes positioned between the anode and the cathode.

35. A reactor according to claim 15 , wherein the separation between any two adjacent electrodes, as measured normal to the face of one of the electrodes, does not vary by more than 30%.

36. A reactor according to claim 15 , wherein the bipolar electrodes are separated by a distance in the range from 0.5 mm to 50 mm.

37. A reactor according to claim 15 , wherein the at least one bipolar electrode has an operational lifetime of 0.2×10 6 Ah/m 2 or more.

38. A reactor according to claim 15 , wherein the at least one bipolar electrode is mounted in a carrier plate which incorporates inlet and outlet manifolds.

39. A reactor according to claim 15 , wherein the at least one bipolar electrode is mounted in an acrylic carrier plate.

40. A reactor according to claim 15 , which further comprises a sensor.

41. A reactor according to claim 40 , wherein the sensor is a COD sensor.

42. A reactor according to claim 15 , further comprising a semi-permeable membrane between at least one pair of adjacent electrodes.

43. An electrode consisting essentially of diamond wherein the diamond comprises a dopant such that the diamond is conductive and has an electrical resistivity of 1 MΩcm or less and wherein the average concentration of dopant in a region of at least one of the major working surfaces, to a depth of 50 nm, is at least 8×10 19 atoms/cm 3 and wherein the electrode has at least one of the following features

a) the concentration of dopant atoms in any 1 mm 3 volume does not vary from the concentration of dopant atoms in any other 1 mm 3 by more than 50%,

b) the uniformity of doping through the thickness of the electrode when measured by SIMS at least five points approximately uniformly spaced through the thickness is such that the maximum dopant concentration is less than about 150% of the mean value and the minimum concentration is greater than about 50% of the mean value,

c) a thickness in the range 0.2 mm to 5 mm,

d) at least one lateral dimension of at least 10 mm, and,

e) a surface area of at least 10 cm 2 .

44. An electrode according to claim 43 , which has a thickness in the range from 0.2 mm to 5 mm.

45. An electrode according to claim 43 , which has a longest dimension of at least 10 mm.

46. An electrode according to claim 43 having a thickness uniformity such that the minimum thickness as measured using the 17-point array technique is more than 60% of the mean thickness and the maximum thickness measured is less than 135% of the mean thickness.

47. An electrode according to claim 43 having an absolute strength measured in three-point bend of greater than 300 MPa.

48. An electrode according to claim 43 , wherein one of the major working surfaces is the as-grown nucleation face.

49. An electrode according to claim 42 , wherein one of the major working surfaces is the as-grown growth face.

50. An electrode according to claim 43 , wherein the first major working surface is the as-grown nucleation face and the second major working surface is the as-grown growth face.

51. An electrode according to claim 43 , which is circular.

52. An electrode as defined in claim 43 , having an operational lifetime of 0.2×10 6 Ah/m 2 or more.

53. A reactor comprising an electrode as defined in claim 43 .