Extreme durability composite diamond electrodes

Abstract

A durable composite diamond electrode is disclosed which comprise at least a relatively thicker conductive UNCD (Ultrananocrystalline Diamond) layer, with low deposition cost, on a substrate underlying a relatively thinner conductive MCD (Microcrystalline Diamond) layer. The electrode exhibits long life and superior delamination resistance under extremely stressed electrochemical oxidation conditions. It is hypothesized that this improvement in electrode reliability is due to a combination of stress relief by the composite film with the slightly softer underlying UNCD root layer and the electrochemically durable overlying MCD shield layer, an effective disruption mechanism of the fracture propagation between the compositing layers, and thermal expansion coefficient match between the diamond layers and the substrate. The diamond composite electrode can be applied to any electrochemical application requiring extreme voltages/current densities, extreme reliability or biomedical inertness such as electrochemical systems to generate ozone, hydroxyl radicals, or biomedical electrode applications.

Claims

1. An electrochemical system comprising an anode containing a first underlying conductive ultrananocrystalline diamond layer prepared by a deposition technique including a methane to hydrogen ratio of at least 2 to about 10 percent and a pressure in the range of about 1 to about 10 Torr that is deposited directly onto at least one side of a substrate and having a first average thickness and a first average grain size that is less than about 100 nm, and an outermost conductive diamond layer overlying the first diamond layer, the outermost diamond layer having a second average thickness and a second average grain size, wherein the second average grain size is more than three times greater than the first average grain size and the average sp.sup.2 content of the first conductive diamond layer is at least five times greater than an average sp.sup.2 content of the outermost conductive diamond layer.

2. The electrochemical system of claim 1, wherein the outermost conductive diamond layer is comprised of microcrystalline diamond or nanocrystalline diamond.

3. The electrochemical system of claim 1, wherein the first conductive diamond layer has an average grain size of less than 20 nm.

4. The electrochemical system of claim 1, wherein the outermost conductive diamond layer has an average grain size of greater than 1 m.

5. The electrochemical system of claim 1, wherein the grain size increases from the first diamond layer to the outermost diamond layer after an interface between the first and the outermost diamond layers.

6. The electrochemical system of claim 1, wherein the first diamond layer is coated on the substrate in a deposition run that includes the outermost diamond layer coated on the first diamond layer, without breaking reactor vacuum.

7. The electrochemical system of claim 1, wherein the first diamond layer is coated on the electrode substrate in a first deposition run followed by the outermost diamond layer coated on the first diamond in a second deposition run separated from the first deposition run.

8. The electrochemical system of claim 1, wherein a resistivity of the first conductive diamond layer is less than 1 ohm-centimeter.

9. The electrochemical system of claim 1, wherein a resistivity of the outermost conductive diamond layer is between 0.001 and 0.1 ohm-centimeter.

10. The electrochemical system of claim 1, wherein the average thickness of the first conductive diamond layer is between 1 and 20 microns.

11. The electrochemical system of claim 9, wherein the average thickness of the first conductive diamond layer is between 2 and 10 microns.

12. The electrochemical system of claim 10, wherein the average thickness of the outermost conductive diamond layer is between 0.5 and 3 microns.

13. The electrochemical system of claim 1, wherein the average thickness of the outermost conductive diamond layer is between 0.5 and 5 microns.

14. The electrochemical system of claim 1, wherein the first conductive diamond layer has an average Young's modulus of less than 900 GPa.

15. The electrochemical system of claim 1, wherein the outermost conductive diamond layer has an average Young's modulus of greater than 900 GPa.

16. The electrochemical system of claim 1, wherein the first conductive diamond layer has an average Young's modulus of less than 800 GPa.

17. The electrochemical system of claim 1, wherein the outermost conductive diamond layer has an average Young's modulus of greater than 1000 GPa.

18. The electrochemical system of claim 1, wherein the average thickness of the first conductive diamond layer is at least two times greater than the average thickness of the outermost conductive diamond layer.

19. The electrochemical system of claim 1, wherein the average thickness of the first conductive diamond layer is at least five times greater than the average thickness of the-outermost conductive diamond layer.

20. The electrochemical system of claim 1, wherein the substrate comprises a non-diamond carbide forming material.

21. The electrochemical system of claim 1, wherein the substrate comprises one or more of niobium, tantalum, tungsten, titanium, molybdenum, zirconium, silicon, silicon carbide, tungsten carbide, pyrolytic carbon or graphite and alloys and mixtures thereof.

22. The electrochemical system of claim 1, wherein both the first and outermost conductive diamond layers are monolithic diamond layers.

23. The electrochemical system of claim 1, wherein an as-deposited average roughness of the first conductive diamond layer is less than 20 nm and an as-deposited average roughness of the outermost conductive diamond is greater than 50 nm.

24. The electrochemical system of claim 1, further comprising at least one additional conductive diamond layer between the first diamond layer and the outermost diamond layer.

25. The electrochemical system of claim 1, wherein the lifetime before delamination failure of the anode at a given current density is at least 5 times greater than the lifetime before delamination failure of a conductive diamond electrode comprised of a single layer of approximately the same thickness as the cumulative thickness of both conductive diamond layers of the anode.

26. The electrochemical system of claim 25, wherein the lifetime before delamination failure of the anode is at least 10 times greater than the time before delamination failure of a conductive diamond electrode comprised of a single layer of approximately the same thickness as the cumulative thickness of both conductive diamond layers of the anode.

27. The electrochemical system of claim 1, wherein the lifetime before delamination failure under constant electrochemical stress of the anode at a current density of 2.5 amps per square centimeter in a predominantly NaCl solution of greater than or equal to 1 molar, is greater than 500 hours.

28. The electrochemical system of claim 27, wherein the lifetime before delamination failure under constant electrochemical stress of the anode at a current density of 2.5 amps per square centimeter in a predominantly NaCl solution of greater than or equal to 1 molar, is greater than 2000 hours.

29. The electrochemical system of claim 1, wherein both the first and the outermost conductive diamond layers are doped with either boron or nitrogen.

30. The electrochemical system of claim 1, wherein the electrochemical system is configured to produce reactive oxygen species such as hydroxyl radicals and/or ozone.

31. The electrochemical system of claim 1, wherein the electrochemical system is configured to produce chlorine and/or hypochlorite.

32. The electrochemical system of claim 1, wherein the electrochemical system is configured to produce peroxodisulphate and/or peroxodicarbonate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic drawing of an embodiment of the invention with a thick underlying polycrystalline diamond layer with a small grain size and an overlying polycrystalline diamond layer with a larger grain size as deposited on an electrode substrate.

(2) FIGS. 2a and 2b are a cross-sectional and top-view SEM micrographs, respectively, of an example of an embodiment of the invention with a 5.9 m underlying layer of conductive UNCD and an overlying 2.0 m layer of conductive MCD as deposited on an electrode substrate.

(3) FIG. 3 is a graphic representation of Typical Highly Accelerated Stress Testing (HAST) voltage versus time testing of prior art 2 m Boron Doped Microcrystalline Diamond (BD-MCD) electrodes in 0.3M acetic acid (HAC) and 0.1 H.sub.2SO.sub.4 at a current density of 0.5 A/cm.sup.2.

(4) FIG. 4 is a schematic drawing of an embodiment of the inventive composite diamond film showing the respective Young's moduli of the two component diamond layers with the underlying BD-UNCD (boron-doped ultrananocrystalline diamond) layer with a lower Young's modulus than the overlying BD-MCD layer (boron-doped microcrystalline diamond) as deposited on an electrode substrate.

DETAILED DESCRIPTION OF THE INVENTION

(5) CVD (Chemical Vapor Deposition) and other diamond deposition techniques including PECVD (Plasma Enhanced Chemical Vapor Deposition) are well known in the art and these prior art techniques can be used to deposit doped diamond with various properties and thicknesses on select electrode substrates. Nitrogen or boron are typically used to dope the diamond for high conductivity. In the data shown here, all of the electrodes were doped with boron and therefore the film type is prefaced with the abbreviation BD (boron-doped). In FIG. 2a, prior art methods of depositions were used to deposit a first underlying (or structural) BD-UNCD layer of approximately 6 m in thickness. As in the prior art, a CH.sub.4/H.sub.2 mixture is used for the deposition with a methane (CH.sub.4) to hydrogen (H.sub.2) ratio of 1-10% and an approximate pressure in the range of 1-10 torr with boron doping gas flow (TMB-Trimethyl Borane, BC.sub.3H.sub.9) with a boron/carbon ratio between 500-12000 ppm. UNCD deposition rates of between 0.1-1.0 m per hour were achieved depending upon the substrate deposition temperature in the range from 400-900 C. This BD-UNCD deposition was followed by an overlying BD-MCD layer deposition of approximately 2 m in thickness. The MCD layer deposition is typically performed with a CH.sub.4/H.sub.2 mixture at a CH.sub.4 to H.sub.2 ratio of 0.1-1% at a pressure in the range of 10-100 torr with a TMB flow of roughly the same as the BD-UNCD deposition. MCD deposition rates for a substrate deposition temperature range from 400-900 C. can be as much as 10 times slower than those of UNCD.

(6) FIG. 1 shows a schematic representation of the inventive composite diamond film as deposited on an electrode substrate wherein the underlying diamond layer exhibits a significantly different grain size than the overlying diamond layer. In an embodiment, the first underlying diamond layer is comprised of BD-UNCD and the second overlying diamond layer is comprised of BD-MCD. Typical thicknesses of diamond layer 1 and diamond layer 2 are in the range of 2-10 m and 1-5 m respectively. Typical grain sizes of diamond layer 1 and diamond layer 2 are less than 10 nm and greater than 100 nm respectively.

(7) The cross-sectional Scanning Electron Micrograph (SEM) of the inventive composite film as deposited on an electrode substrate shown in FIG. 2a clearly shows the underlying (structural) UNCD layer deposited on a smooth silicon wafer substrate (as example of an electrode substrate) and the overlying (functional) MCD layer deposited on top of the UNCD layer.

(8) FIGS. 2a and 2b present an SEM images of (i.e., left2a) cross-sectional view and (i.e., right2b) top view, respectively, of an embodiment of the inventive composite diamond electrode. An underlying 5.9 m thick BD-UNCD film with an average grain size of less than 10 nm is covered by a MCD film of approximately 2.0 m in thickness. From the top view it can be seen that the BD-MCD layer has an average grain size of between approximately 0.2 m and 2 m.

(9) FIGS. 2a and 2b present an SEM images of (i.e., left2a) cross-sectional view and (i.e., right2b) top view, respectively, of an embodiment of the inventive composite diamond electrode. An underlying 5.9 m thick BD-UNCD film with an average grain size of less than 10 nm is covered by a MCD film of approximately 2.0 m in thickness. From the top view it can be seen that the BD-MCD layer has an average grain size of between approximately 0.2 m and 2 m.

(10) The inventive conductive composite diamond electrode was tested under Highly Accelerated Stress Test (HAST) conditions at high current density and with varying levels of chemical acceleration. The literature (e.g. Kraft, Doped Diamond: A Compact Review on a New, Versatile, Electrode Material, Int. J. Electrochem. Sci., 2(2007) 355-385, p. 363), indicates that acetic acid (C.sub.2H.sub.4O.sub.2) or HAC is highly effective at accelerating the electro-etching of doped diamond. Current density acceleration is a standard HAST technique for electrode testing in our lab and has indicated that lifetimes before delamination decrease in rough proportion to the cube of increasing current density over a range from 2.0-3.0 A/cm.sup.2. Extrapolation of the lifetime to normal operating conditions was estimated to be a reasonable and conservative verification of HAST testing. Testing of previous single layer diamond electrodes (see Table 1) have indicated that this is a conservative estimate and that actual lifetimes to delamination at typical operating current densities, e.g. 0.15-0.50 A/cm.sup.2, are in fact longer than would be predicted from a simple cube-law extrapolation. However, this extrapolation will be used in this invention since the current density typical of ozone applications, i.e. 1.0-2.0 A/cm.sup.2, is much closer to the HAST conditions used for test to failure of the inventive electrodes and therefore the extrapolation will be less likely to diverge from the fitted cube law estimation.

(11) Acetic Acid Testing:

(12) An extreme chemical acceleration of the delamination/electro-etching of the diamond electrodes was achieved with an aqueous solution comprising 0.3 M HAC, 0.1 M sulfuric acid (H.sub.2SO.sub.4), 0.1 M sodium perchlorate (NaClO.sub.4). The diamond coated electrodes were exposed to this solution while being subject to a constant current density (in galvanostatic mode) of 0.5 A/cm.sup.2 at roughly 40 C. (104 F.), with an electrode gap of roughly 1 cm, and an applied voltage varying in a range between 10-25V. Multiple electrodes were tested using this HAST protocol with a lifetime to failure defined by an 3V increase in applied voltage which corresponded to an approximate delamination percentage of the diamond film electrode of 90-95% as observed in an optical microscope. The inventive electrodes with composite diamond film (5.5-m thick BD-UNCD covered by 2-m thick BD-MCD), tested with this method exhibited lifetime to delamination failure in the range of 50 to 60 Ahr/cm.sup.2 (100-120 hours under test at 0.5 A/cm.sup.2). See Table 1 for details. Testing using the same HAST protocol applied to previous generations of electrodes with a single layer of diamond deposited on the substrate, produced lifetimes to failure of roughly 5 to 7 Ahr/cm.sup.2 (10-14 hours under test). Therefore the inventive conductive composite (2 layer: UNCD MCD) diamond electrodes as shown in FIG. 4, exhibited lifetimes to delamination failure of approximately 10 times greater than previous single layer electrodes using this extreme chemical acceleration HAST protocol.

(13) Sodium Chloride (Salt Solution) Highly Accelerated Stress Testing:

(14) A similar test regime was employed to that described above for HAC except that a 1M solution of sodium chloride (NaCl) was employed instead of HAC and the applied current density was much higher (2.5 A/cm.sup.2). Other test conditions were the same as those listed above. Lifetimes to failure were as high as 8000 Ahr/cm.sup.2 (corresponding to a time under test of >3200 hours) (see Table 1 for details). Such a test protocol was conducted before the HAC protocol, and is very time and labor costly, so we eventually used HAC protocol to evaluate the durability of electrodes. If a cube-law current density function of delamination lifetime is applied to this result for inorganic electrochemistry (e.g. solutions of NaCl or H.sub.2SO.sub.4), these lifetimes would correspond to a time to delamination failure of >342 years at a current density of 0.25 A/cm.sup.2, >58 years at 0.45 A/cm.sup.2 and >5 years at 1.0 A/cm.sup.2. The actual lifetime of a single layer conductive diamond electrode being operated at 0.45 A/cm.sup.2 has now been confined to be in excess of 2.0 years with only 60% delamination (i.e. it has not yet failed) which when compared to the cubed-law dependence would have failed at roughly 1.0 year, demonstrates that these single layer conductive diamond electrodes (and likely the inventive two layer conductive diamond electrodes) would last longer than a cube-law extrapolation as a function of current density would imply at these more typical current density operating conditions.

(15) TABLE-US-00001 TABLE 1 Highly Accelerated Stress Testing (HAST) on Diamond Electrodes with various thicknesses of Boron-Doped Ultrananocrystalline Diamond (BD-UNCD) and Boron-Doped Microcrystalline Diamond (BD-MCD) in current density accelerated and chemically accelerated HAST conditions: HAST in 0.3M HAC, 0.1M HAST in 0.3M H.sub.2SO.sub.4 at 0.5 A/cm.sup.2, 40 C. HAC, 0.1M H.sub.2SO.sub.4 Diamond Deposition HAST in 1M NaCl (individual at 0.5 A/cm.sup.2, 40 C. Parameters at 2.5 A/cm.sup.2, 40 C. measurements) (average) 2.0 m thick BD-UNCD <100 Ahr/cm.sup.2 (avg) Dies almost immediately ~0 5.0 m thick BD-UNCD 200-500 Ahr/cm.sup.2 Dies almost immediately 0 (avg) 2.0 m thick BD-MCD ~500 Ahr/cm.sup.2 (avg) 6.8, 3.8, 4.6, 5.2, 5.3, 5.6 5.2 Ah/cm.sup.2 Ahr/cm.sup.2 4.5 m thick BD-MCD 7678 Ahr/cm.sup.2 17.5, 19.0 Ahr/cm.sup.2 18.2 Ah/cm.sup.2 INVENTION: 5.0 m thick BD- >8000 AHr/cm.sup.2 45.5, 47.5, 56.3, 50.3, 48.5 49.6 Ah/cm.sup.2 UNCD + 2 m BD-MCD (still under test) Ahr/cm.sup.2

(16) Table 1 presents representative data comparing the HAST under current density acceleration only (2.sup.nd column in Table 1) in 1M NaCl (58 g/L) at an extreme current density of 2.5 A/cm.sup.2 as compared to (3.sup.rd and 4.sup.th columns) both a mild current density acceleration (0.5 A/cm.sup.2) and an extreme chemical acceleration in 0.3 M HAC, 0.1 H.sub.2SO.sub.4. HAC provides a much more extreme HAST condition. Given the lower current density an approximate calculation of the extra acceleration by HAC can be made from the 2.sup.nd, 3.sup.rd and 4.sup.th row of comparative data. This is not definitive, but the HAC acceleration factor is likely to be at least 10,000 times greater than current density alone, i.e. in 1M NaCl. The innovative composite diamond electrodes were so reliable to delamination failure in 1M NaCl alone, that lifetime testing was restricted to the chemical HAST conditions to allow actual times to delamination failure of less than 6 months. Using a very conservative acceleration factor based upon the cube law extrapolation and the 10,000 times chemical acceleration factor calculated approximately from the other rows in the table, which is itself conservative, the bottom row of Table 1 (5 m BD-UNCD/2 m BD-MCD) would be expected to last more than 10 years at 1 A/cm.sup.2 under non-chemical acceleration conditions. 1 A/cm.sup.2 is an extreme current density for many electrochemical applications. It should be noted that the inventive film 5 m BD-UNCD and 2 m BD-MCD electrode has a shorter deposition time (less expensive) than the single layer 4 m prior art BD-MCD film also listed in Table 1. Note, all of these results were derived from HAST measurements of planar electrodes. The non-planar geometry of typical ozone electrodes (e.g. cylindrical holes in a Nb substrate coated with diamond) would be expected to experience localized areas of higher electric fields, which could lower the lifetime for ozone generation or other non-planar geometry electrode applications. Higher HAST lifetimes of planar diamond films are required (or recommended) to accommodate the reliability requirements of these more extreme conditions.

(17) FIG. 2b presents a top view SEM of the inventive composite diamond electrode with BD-MCD gain formation clearly evident with a variable grain size in the approximate range of 0.2-2 m whose HAST data is shown on the bottom line of Table 1. The diamond electrode comprising the underlying BD-UNCD layer and the overlying BD-MCD layer would exhibit an average roughness in the range of 20-100 nm if deposited on a smooth electrode substrate, such as a silicon wafer with typical average roughness in the range of 0.2-0.3 nm. A thicker film of MCD would increase the grain size and the roughness of the composite film and significantly increase the deposition time and cost. However, this is unnecessary since the inventive electrode delivers improved reliability results even with the faster deposition times conferred by the significant thickness of underlying structural BD-UNCD as deposited on an electrode substrates.

(18) FIG. 3 presents voltage versus time HAST data for four prior-art 2 m thick BD-MCD electrodes and one set of data for the inventive longer life composite diamond electrode. This testing was conducted in the extreme chemical HAST conditions described above, i.e. 0.2 M HAC plus 0.1 M H.sub.2SO.sub.4 at a current density of 0.5 A/cm.sup.2 and a temperature of 40 C. This data is shown to illustrate the HAST method used to generate the data shown in Table 1 above. There is considerable scatter in the data but the overall average of these particular four electrodes is 7 Ahr/cm.sup.2. The overall average for all the data for these prior art electrodes is about 5.6 Ahr/cm.sup.2. The higher lifetime inventive composite diamond electrode that is shown on the same scale does not exhibit any increase in voltage at the end of the trial (10 Ahr/cm.sup.2 or 20 hours of testing at 0.5 A/cm.sup.2). The lifetime of the inventive films would not be visible on the scale of this graph reflected in the data from Table 1 given their considerably longer HAST lifetimes even under these extreme chemical acceleration conditions.

(19) FIG. 4 presents a schematic image of the inventive composite diamond electrode characterizing the differential Young's modulus between the underlying BD-UNCD and the overlying BD-MCD layers. The Young's modulus of the underlying UNCD layer is less than 900 GPa and the Young's modulus of the overlying MCD layer is greater than 900 GPa. Typical BD-UNCD Young's modulus can be in the range of 550-900 GPa and can be adjusted by adjusting the deposition parameters. The Young's modulus of BD-MCD is closer to that of single crystal diamond (1220 GPa) and is typically in the range of 900-1200 GPa. The combination of the extreme chemical affinity between a BD-MCD diamond layer grown on an existing BD-UNCD layer with the nearly identical linear thermal expansion coefficient between the two layers, i.e. 1 ppm, provides nearly ideal adhesion between the two diamond layers.

(20) It is well known to those skilled in the art of thin film deposition on metal or silicon electrodes that the use of strain-relieving layers can dramatically impact the quality of additional thin films grown on top of such layers. This is particularly true for the integration of epitaxial layers with substrates in which there is a significant lattice mismatch between the overlayer and electrode substrate. So-called buffer layers are used to distribute the stress within the heterostructure to prevent delamination and improve the overall material properties of the overlayer. An underlying diamond layer of BD-UNCD therefore serves the purpose of a buffer layer to distribute the deposition stress and stress generated in the layer during usage and thereby improve the overall delamination resistance of the composite film under shear stress which is particularly severe during high current density electrochemical oxidation (anodic oxidation) of a metallic, silicon or dielectric electrode substrates coated with doped diamond.

(21) Without intention of being bound by a particular theory, it is hypothesized that the combination of the strong adhesion between the two diamond layers as deposited on an electrode substrate and the cushioning effect of the somewhat softer underlying buffering BD-UNCD layer provides some of the observed improvement in delamination resistance under shear stress caused by electrochemical oxidation. Additionally, the discontinuity in grain size between the two diamond layers as deposited on an electrode substrates may contribute to a reduction in defect propagation probabilities. Not withstanding the complex potential mechanisms that may contribute to the overall improvement in durability to shear stress, the experimental data indicates an improvement in lifetime under these extreme shear stress conditions of at least 5-10 times over non-composite BD-MCD films of the same or similar thickness as deposited on an electrode substrates. The extreme shear stress under high voltage/current density electrochemical oxidation is sufficient to pulverize even diamond films over time. However, BD-MCD exhibits larger grain sizes and Young's moduli and is therefore expected to exhibit greater resistance to this oxidative shear stress. However, such thick BD-MCD films would be much more expensive to deposit on an electrode substrate, due to their 340 times longer deposition times. The increase in reliability for a given thickness of the inventive composite film therefore offers the advantage of a thinner and less expensive composite diamond electrode for a given application and target reliability.

(22) Alternative embodiments of the inventive diamond electrodes include the use of doped nanocrystalline diamond as the underlying layer and BD-MCD as the overlying layer or BD-UNCD as the underlying layer and BD-MCD as the overlying layer. The use of only two such layers may be sufficient for most applications. However, where extreme reliability or thicker diamond layers are appropriate (e.g. for ozone electrodes or high temperature applications), an additional set of underlying and overlying layers may be appropriate. This could involve a third diamond layer similar in properties (but not necessarily thickness) as the first diamond layer, (e.g. BD-UNCD) and a fourth diamond layer similar in properties (e.g. BD-MCD) to the second overlying layer.

(23) Biomedical applications are appropriate for these composite diamond electrodes given their extreme hardness, bioinertness, chemical resistance and extreme reliability. Such applications could include cardiovascular devices, and other electrochemical or electrode implantables where these extraordinary properties would present an advantage over the prior art. For example, automatic defibrillators require a form of heart surgery to replace batteries. The battery lifetime is severely limited by the build-up of the body's immune system at or near the point of contact between the electrode and the surrounding tissue. The well-known extreme chemical and bioinertness of composite diamond electrodes could present a significant advantage in reducing the body's immunological reaction to the presence of these implantables and should significantly lengthen the useful lifetime of battery power for these devices.

(24) These and other variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications.