Method for making poly(hydridocarbyne)

The invention relates to a method for making a poly(hydridocarbyne) polymer for use as a ceramic precursor for creating adhesive, thin, cost-effective diamond or diamond-like carbon (DLC) coatings over large, complex surfaces. Said coatings act as a highly corrosion-resistant barrier, and can be doped to be electrically conductive.

Diamond and diamond-like carbon (DLC) coatings have great commercial utility for use as protective coatings, especially for electrode coatings, since such coatings can be doped to be electrically conductive, and are highly corrosion resistant even in strong oxidizing solutions in electrochemical applications with high overvoltage (see for example, M. Fryda, Th. Matthee, S. Mulcahy, A. Hampel, L. Schafter, I. Troster, Diamond and Related Materials, 12 (2003) 1950-1956).

Natural diamond is generally comprised of a cubic crystalline form of carbon. DLC, which is a mixture of mostly sp 3 —hybridized amorphous carbon, has some of the properties of diamond. Historically, DLC has been applied as a protective coating using a chemical vapour deposition (CVD) process, which process is expensive and limited to coating of planar and relatively small, simple surface topographies.

Thus, the discovery by Patricia A. Bianconi, et. al. (J. Am. Chem. Soc., 2004, 126(10), 3191-3202) of the pre-ceramic precursor polymer poly(hydridocarbyne) (PHC) as a source material to form diamond and DLC coatings at atmospheric pressure and relatively low temperature pyrolysis represents a significant new class of material. PHC is a unique polymer which is a structural isomer of polyacetylene, but with a sp 3 —hybridized, tetrahedrally bound carbon network backbone comprised of [CH] n . Each carbon must contain a hydrogen substituent to prevent conversion to sp 2 carbon so as to minimize the ratio of sp 2 to sp 3 carbon. Bromoform, which has an sp 3 tetrahedral structure was used as a starting material.

Thermal decomposition of PHC results in lonsdaleite, a hexagonal form of diamond. Poly(hydridocarbyne) (PHC) is an air-stable solid at room temperature that forms a nanoparticle or colloidal-dispersion in many polar organic solvents. This feature allows for simple, low cost “dip-coatings” of PHC-organic solvent solutions over large, complex surfaces, where said solution can be dried and heated to form adhesive diamond and DLC sub-micron to micron-thick corrosion resistant thin films or conformal coatings.

Thus, there is great interest and utility in synthesizing PHC cost-effectively on a commercial scale.

Additionally, PHC can be used as a precursor material for synthesizing other materials such as, for example, graphite-like nanospheres (see S. Xu, X. B. Yan, X. L. Wang, S. R. Yang and Q. J. Xue, J. Mater. Sci. (2010), 45:2619-2624). The decomposition of PHC can also be used to increase the tensile strength in an exfoliated graphite matrix (see D. V. Savchenko, S. G. Ionov and A. I. Sizov, Inorganic Mat. (2010) vol. 46 (2):132-138.

As carbon, diamond and DLC coatings are highly biocompatible, the application of DLC coatings over implants such as stents, eye and brain electrodes, cochlear devices, pacemakers, defibrillators, and hip, knee, etc. prosthetics is advantageous.

Since such conformal coatings can also be converted to diamond and DLC by light-activation (i.e. UV laser) processes, new applications such as protective DLC coatings over teeth for dental treatment is possible.

DLC coatings can also be made electrically conductive by doping with, for example, nitrogen (i.e. see A. Zeng, E. Liu, S. N. Tan., S. Zhang, J. Gao, Electroanalysis 2002, 14, No. 15-16, pp. 1110-1115), boron (i.e. see M. Fryda, Th. Matthee, S. Mulcahy, A. Hampel, L. Schafer, I. Troster, Diamond and Related Materials 12 (2003) 1950-1956) or aluminum (i.e. see N. W. Khun, E. Liu, J. Nanoscience and Nanotechnology 2010, 10(7), pp 4767-4772).

Alternatively, PHC can be used as a convenient source material for conventional CVD deposition of diamond and DLC coatings by simple heating without addition of hydrogen or an activation procedure. The traditional method of deposition of tetrahedral amorphous carbon contains a portion of sp 2 carbon, which tends to contaminate the final CVD deposited diamond or DLC film. However, by controlling the formation of sp 2 carbon, such carbon bonding provides for in-situ doping, thereby providing for electrical conductivity.

U.S. Pat. No. 5,516,884 to Bianconi teaches the formation of 3-D tetrahedrally hybridized carbon-based random network polymers where elements such as silicon, germanium, tin, lead and lanthanides can be incorporated into the network backbone. Each carbon atom has one substituent and is linked via three carbon-carbon single bonds into a 3-D network of continuous fused rings. Thermal decomposition of such polymers forms diamond and DLC carbon. Specifically, Bianconi describes the method for making a variety of polycarbynes, including the synthesis of poly(phenylcarbyne-co-hydridocarbyne) in a 99:1 ratio. However, part of the synthesis process involves the use of NaK alloy, an extremely pyrophoric material, under ultrasonic irradiation, in an inert atmosphere, plus various organic solvents, and additional processing and filtration steps, including refluxing with methyl lithium, rendering this low-yield approach highly problematical for commercial production.

Huang et al. (S. M. Huang, Z. Sun, C. W. An, Y. F. Lu and M. H. Hong, 2001, J. App. Phy., 90(5), 2601-2605) use the reductive condensation of a 1,1,1-tricholorotoluene monomer with an emulsion of NaK alloy in tetrahydrofuran under inert atmosphere to synthesize poly(phenylcarbyne). They also provide data on using a pulsed UV laser for converting the poly(phenylcarbyne) to a diamond-like structure.

U.S. Pat. No. 6,989,428 to Bianconi, et al. provides a detailed summary of the prior art for polycarbyne ceramic polymers used to form diamond and diamond-like carbon. Specifically, it discloses details for the synthesis of poly(methyl- and ethyl-silyne) as a precursor of silicon carbide. However, such synthesis again involves a pyrophoric alloy such as NaK, plus a plethora of processing steps involving various organic solvents and long multiple refluxing steps.

Jung-Hwan Hah, et al. in US Patent Application 2006/0115772 A1, in one embodiment, teach the preparation of poly(hydridocarbyne) by the reductive coupling of CH n X 3 4-n , where X 3 includes a group VII halogen such as fluorine, chlorine, bromine or iodine, and n is an integer from 1 to 3. Since each halomethane requires one substituent hydrogen to prevent conversion to sp 2 carbon, the Hah reference to n=2 and n=3 suggests that these versions would not form poly(hydridocarbyne). They describe dissolving the poly(hydridocarbyne) in an organic solvent to create a polymeric film for forming a hard mask for fine pattern photolithographic applications. Their reductive coupling step still requires a metallic compound such as NaK or methyl lithium, and in some embodiments heat, ultrasonic wave, light or combinations thereof. No further details are disclosed. However, their process appears very similar to that described by Bianconi in U.S. Pat. No. 5,516,884.

Recent developments by Yusuf Nur et al. (Yusuf Nur, Michael W. Pitcher, Semih Seyyidoglu and Levent Toppare, J. Macromolecular Science, Part A, 2008, 45(5), pp 358-363) and US Patent Application 2010/0063248 A1, describe a method for making PHC using the electrochemical polymerization of chloroform. Said approach is simpler, and potentially safer, than that given in U.S. Pat. No. 5,516,884 by Bianconi.

The Nur process involves electrolyzing chloroform in the presence of acetonitrile, with tetrabutylammonium tetrafluoroborate as electrolyte, run at −6V under nitrogen for 4 hours at room temperature. Various additional steps (i.e. refluxing with tetrahydrofuran and LiAlH 4 for 12 hours), plus final PHC purification steps using dichloromethane and hexane are required. Said process also produces an undefined “insoluble material”, and generates chlorine gas, with PHC produced at a 30-40% yield.

In a subsequent publication Nur et al. (Yusuf Nur, Halime M. Cengiz, Michael W. Pitcher and Levent K. Toppare, J. Mater. Sci. 2009, 44: 2774-2779) describe the electrosynthesis of PHC from hexachloroethane. The method used was substantially the same as that using chloroform, with the key difference being that the PHC polymer chain length is bigger using hexachloroethane as a starting material. The methods described by Nur et al. using chloroform and hexachloroethane as starting materials, are still complex, and problematical for producing PHC commercially.

Their method operates the electrochemical cell at −6 V, which exceeds the decomposition voltage of most organic solvents (which tend to decompose at less than 3 volts) thus forming unwanted by-products, including toxic gas such as chlorine gas.

The present invention overcomes the prior art limitations for the synthesis of poly(hydridocarbyne).

The inventive process for making poly(hydridocarbyne) (PHC) uses at least one active-metal electrode immersed in a trisubstituted halomethane or hexachloroethane solution containing a conductive electrolyte in an electrochemical cell. A relatively small potential is applied between the electrodes of the cell to produce PHC at one or both of the electrodes, in solution or precipitated from solution.

The active-metal forms part of, and partakes, in the reaction within the electrochemical cell to form PHC, the active-metal/halomethane salt and an inorganic halide acid (i.e. HF, HCl, HBr and HI). Depending on the reaction conditions, there may also be the formation of additional parasitic species such as acetylene, polyacetylene, hexahalobenzene, etc. Additionally, the active-metal electrode can react to form a halide salt and other reaction products, which products may coat the electrode surface slowing the reaction process, requiring periodic cleaning of said electrode surface.

Some of the possible precursor trisubstituted halomethanes are: CHF 3 , CHClF 2 , CHBrF 2 , CHBr 2 F, CHCl 3 , CHBrCl 2 , CHBr 2 Cl, CHBr 3 , and CHI 3 . Combinations of said halomethanes can also be used. Four simple, readily available, trisubstituted halomethanes are CHF 3 (fluoroform), CHCl 3 (chloroform), CHBr 3 (bromoform) and CHI 3 (iodoform), all of which have an sp 3 carbon bond tetrahedral structure.

Hexachloroethane, which also has an sp 3 tetrahedral structure comprised of two tetrahedra sharing a common edge, can also be used as a PHC precursor material, but it must first be dissolved in a trisubstituted halomethane solution, or another solvent (such as acetonitrile), as hexachlorethane is a solid at room temperature, sublimes at about 187° C. and does not contain a hydrogen.

Suitable active-metals for the electrode include metals such as lithium, sodium, potassium, magnesium, calcium, zinc, aluminum and silver. It should be noted that certain active-metals such as lithium, sodium, potassium, calcium, magnesium, zinc, powdered aluminum, etc. can react spontaneously and explosively with some trisubstituted halomethanes and hexachloroethane, requiring extreme caution in handling and using these materials.

The active-metal electrode is selected such that is not quite sufficiently reactive to react spontaneously with the halomethane (or hexachloroethane) solution, but rather requires an electrical current to induce such reaction. The voltage applied to the electrodes to initiate the electrochemical reaction is minimized to avoid decomposing the electrolyte, any solvent and halomethane (or hexachloroethane) solution as such decomposition can form various parasitic species. It is advantageous to use the minimum voltage necessary to form PHC. In cases where a solvent (such as acetonitrile) is added to aid in dissolution of any electrolyte species, a voltage less the 3 volts is preferred. Use of higher voltages (e.g. 3-10 volts) will tend to decompose the electrolyte, solvent and halomethane/hexahaloethane, which decomposition can form various parasitic species.

The voltage applied to the electrochemical cell can be DC, where the DC voltage is continuous, pulsed, shaped, with a fixed or variable duty cycle, and where such voltage is less than 10 volts, preferably less than 5.5 volts, preferably less than 3 volts, preferably less than 3 volts. The voltage applied to the electrochemical cell can be AC, where said AC voltage is frequency modulated, chopped or pulse shaped, and where such voltage is less than 10 volts (RMS or peak), preferably less than 5.5 volts (RMS or peak), preferably less than 3 volts (RMS or peak).

If the voltage is DC, only one electrode will partake in the electrochemical reaction to form PHC. If it is AC, both electrodes (assuming they are both active-metals) will partake in the electrochemical reaction to form PHC as the polarity switches back and forth.

By controlling the characteristics of the AC or DC voltage applied to the electrodes during synthesis, the electrochemical reaction for the formation of PHC can be optimized. The use of large surface area electrodes (i.e. maximizing the electrode surface to solution volume) is also preferred, so as to optimize the production of PHC. The use of porous electrodes where the geometric (i.e. footprint) surface area of the electrode is less than the actual surface area is preferred.

In order to provide electrical conductivity to initiate the electrochemical reaction, a seed amount of an appropriate substance may be added to the cell solution. This may comprise a seed amount of the active metal halide salt, an inorganic halide acid or an intrinsically conductive polymer or its salt derivative. A suitable solvent may be required.

By using only a limited number of reactants, and a simple electrochemical process, the inventive process described herein is direct, definable and avoids, or minimizes, parasitic side reactions and toxic gas production. The invention is a relatively low-cost, direct electrochemical method for making poly(hydridocarbyne) using readily available, simple precursor chemicals.

The foregoing was intended merely as a summary and was not intended as an exhaustive narration of the features of the invention, which are more properly appreciated in the context of this entire specification, including the claims.

According to the preferred embodiment, the invention uses a hybrid combination of an active-metal electrode and electrochemical means to form poly(hydridocarbyne) using one or more of a trisubstituted halomethane (CHX 3 , where X is fluorine, chlorine, bromine or iodine, preferably chlorine or bromine), with an sp 3 tetrahedral structure.

The trisubstituted halomethanes HCF 3 , HCCl 3 , HCBr 3 , ICBr 3 are one source of sp 3 carbon for starting material for the synthesis of PHC polymer via active-metal electrolysis. Other trisubstituted halomethanes that are sources of sp 3 carbon are CHF 3 , CHClF 2 , CHBrF 2 , CHBr 2 F, CHCl 3 , CHBrCl 2 , CHBr 2 Cl, CHBr 3 , and CHI 3 .

Alternatively, the trisubstituted halomethane may be replaced by hexachloroethane dissolved in a trisubstituted halomethane solution, such as fluoroform, chloroform, bromoform or iodoform (preferably chloroform or bromoform) or in an organic solvent (preferably acetonitrile).

Since halomethanes are not electrically conductive, electrolyte species such as halide salts, intrinsically conductive polymers or their salt derivatives, and/or inorganic halide acids may be seeded into the solution to provide electrical conductivity to initiate the electrochemical reaction. A solvent, preferably a polar solvent such as acetonitrile, may be added to the trisubstituted halomethane solution to aid in the dissolution of the electrolyte species.

Electrolytes can also generate parasitic compounds, especially if higher voltages are used during electrolysis. Preferably, the electrolyte(s) have a higher over-potential than the halomethane solution or any additional solvent used.

The intrinsically conductive polymer can be such polymers as polyaniline, polyaniline nanofibers, polyparaphenylene, polyacetylene, polyphenylene-vinylene, polypyrrole, polythienylene-vinylene, polytriphenylamine, etc.

If an inorganic halide acid is used as the electrolyte species, it is preferably a non-aqueous inorganic halide acid such as HF, HCl, HBr or HI. Since HF, HC, HBr and HI are gases at room temperature and atmospheric pressure, such gases require dissolution into the electrolysis cell solution, preferably via bubbling.

Alternatively a polyaniline conductive salt (i.e. the emeraldine base) may be added to the halomethane solution to provide electrical conductivity. The addition of N-methyl pyrrolidinone may aid in the dissolution of polyaniline and the emeraldine base.

Equation 1 below represents the basic reaction of the invention. Specific trisubstituted halomethane solutions and reactive-metals will form additional species, not shown in Equation 1 (i.e. dimers such as Al 2 Br 6 ). Also, the formation of possible parasitic products, such as acetylene, polyacetylene, hexahalobenzene, etc. are not shown. Additionally, the formation of the halide acid is not shown in the equation.

nHCX 3 (solution)+nM (electrode)→nMX m (electrolyte)+[HC] n (PHC polymer)  (Eq. 1)

where:

X 3 is F 3 or Cl 3 or Br 3 or I 3 M is an active-metal such as lithium, sodium, potassium, magnesium, calcium, zinc, aluminum, silver, etc. In one embodiment, the metal M is to comprised of an alloy containing one or more active-metals. MX m is the metal halide salt formed from fluorine, chlorine, bromine or iodine (i.e. LiF, NaF, MgF 2 , CaF 2 , ZnF 2 , AlF 3 , AgF; LiCl, NaCl, MgCl 2 , CaCl 2 , ZnCl 2 , AlCl 3 , AgCl; LiBr, NaBr, MgBr 2 , CaBr 2 , ZnBr 2 , Al 2 Br 6 , AgBr; LiI, NaI, MgI 2 , CaI 2 , ZnI 2 , AlI 3 , AgI) a seed amount of an intrinsically conductive polymer, a conductive polymer salt, a halide salt or an inorganic halide acid is added as an electrolyte. [HC] n is the PHC polymer with “n” being an integer denoting the chain length the voltage of the electrochemical reaction is minimized to reduce the production of parasitic species the voltage format i.e. AC, DC, pulse length, frequency etc. are optimized to minimize the formation of sp 2 carbon, depending at least in part on the reactivity of the active-metal that is selected the rate of reaction is maximized so as to minimize the formation of sp 2 carbon the reaction is conducted in an inert atmosphere the temperature of the reaction is run at room temperature, or at a temperature where the trisubstituted halomethane solution is a liquid the trisubstituted halomethane solution is anhydrous and is a liquid at atmospheric pressure

In one embodiment, the voltage applied for Equation 1 is DC, where such DC voltage can be continuous, pulsed, shaped, with a fixed or variable duty cycle. In another embodiment, the voltage applied for Equation 1 is AC, where such AC voltage can be frequency modulated, chopped or pulse shaped. Such features are advantageous to optimize the rate of reaction, the PHC polymer chain-length, ring size and particle size. The selection of the specific type of waveform and the voltage that will achieve an optimal yield will depend on the particular active-metal and electrolyte being used. However, the exemplary embodiments detailed below produce acceptable yields.

In the preferred embodiment, both the positive and the negative electrodes are comprised of an active-metal material or alloy thereof. In another embodiment, only one electrode is comprised of an active-metal. In yet another embodiment, the container walls comprise one of the electrodes. It is preferable to create at least one large surface area electrode, and to maximize the surface area of the electrode to the volume of the halomethane solution to create the maximum reactive surface area. Use of porous electrodes, or porous materials coated with reactive-metals is advantageous.

In another embodiment, the active-metal electrode contains at least one non-active-metal dopant material such as carbon, nitrogen, boron or aluminum to provide for electrical conductivity in the resultant diamond or DLC coating made from pyrolized PHC.

In yet another embodiment, the one or more electrically-conducting dopant materials such as carbon, nitrogen, boron or aluminum are added to the halomethane solution during electrolysis to provide for electrical conductivity in the resultant diamond or DLC coating made from pyrolized PHC.

An inert gas, preferably helium, argon or xenon, can be intermittently or continuously bubbled through the electrochemical cell during electrolysis to strip-out or remove any parasitic gases that are generated. However, it should be realized that such stripping process may also remove some of the dissolved inorganic halide acids, which acids can provide beneficial electrical conduction during electrolysis.

The halomethane solution and active-metal must be carefully selected such that the halomethane does not react spontaneously and or explosively either before or during the electrochemical reaction. Active-metals such as lithium, sodium, potassium, magnesium, calcium, zinc or powdered aluminum may also react explosively with many trisubstituted halomethane solutions, thus great care is required in handling reactions involving said materials. Similarly, the choice of active-metals is limited due to the explosive and highly reactive nature of some of these metals with trisubstituted halomethanes.

Various examples of exemplary reactions are shown below. These examples are indicative of various possible combinations, and as such, are not meant to be limiting.