Fabrication of titanium and titanium alloy anode for dielectric and insulated films

A method for constructing a titanium film capacitor. The method includes fabricating an anode from at least one of titanium and a titanium alloy and forming a homogeneous anode film on at least one surface of the anode.

BACKGROUND OF THE INVENTION

The present invention relates generally to capacitors and more particularly to systems and methods for the fabrication of titanium and titanium alloy anodes for dielectric and insulated films.

Titanium and titanium alloy anodes have an advantage in energy density (J/g), cost ($/g) and material density (g/cm3) when compared with tantalum and niobium anodes. Further, titanium is capable of providing a higher capacitance·voltage per gram (CV/g), for example, the gain is up to 10 times that of tantalum. Thus, a capacitor fabricated with a titanium or titanium alloy anode is physically smaller or, alternatively, a like sized component, possesses a higher capacitance rating than that fabricated with tantalum. When titanium or a titanium alloy is used instead of tantalum in capacitor fabrication in a capacitor having similar performance specifications, material costs are also greatly reduced, by as much as 100 times.

While titanium fabrication is advantageous, physical characteristics of the metal have heretofore prevented widespread adaptation. The structure, crystallization kinetics, and composition, as well as the electrical and electrochemical behavior, of an anode film on titanium or a titanium alloy are somewhat contradictory and inconsistent in nature. Moreover, these phenomena are mutually and multifariously related during anodizing. Further, the insulating and dielectric behavior of the anode film, e.g., leakage current and capacitance, are uncertain and inconsistent irrespective of the electrochemical parameters, including voltage, current, chemistry, concentration of electrolyte, heat treatment, and the like. As a result, titanium and titanium alloys have generally been disqualified from use in capacitors, save in possible exceptions metal for certain metal capacitors.

Thus, there exists a need resolve the inconsistent capacitance and the high and variable leakage current behaviors associated with the use of titanium and titanium alloys for anodes. Furthermore, there exists a need for controlling leakage current and capacitance in the processing or manufacture of capacitors using titanium and titanium alloy anodes. Also, in order to use titanium or a titanium alloy as a capacitor anode, the origin of the inconsistency and variation in the electrical behavior of the anode film must be understood. Moreover, the critical factors in processing or manufacture should be clearly defined. Thus, there exists a need for a new fabrication technique for anodes using titanium or a titanium alloy. Low equivalent series resistance (ESR) and high heat dissipation are also generally desired.

SUMMARY OF THE INVENTION

The present invention teaches a system for constructing a titanium film capacitor, including a capacitor constructed therefrom. The method includes fabricating an anode from at least one of titanium and a titanium alloy and forming a homogeneous anode film on at least one surface of the anode.

The system teaches further shot peening the anode to heavily deform a surface of anode. Such deformation of the anode enhances migration of titanium ions at the interface between the anode and the anode film.

The system also teaches quenching the anode so that the anode has an amorphous structure. The amorphous nature of the anode supplies a higher density of active sites at which to nucleate titanium oxide relative to a crystalline structure.

The system also teaches depositing an anode film using calcium, calcium oxide, manganese, manganese oxide, magnesium, magnesium oxide or a combination thereof.

The method further teaches etching the anode to roughen the surface of the anode. Such a surface roughness preferably has a height from approximately 1 nanometer (nm) to approximately 0.1 millimeter (mm) and a width from approximately 1 nm to approximately 0.1 mm.

The system also teaches the use of sputtering to form the anode film on the anode. Such sputtering is preferably performed using an argon gas.

The system also teaches forming the anode film through treating the anode with one of hydrogen (H), the alkali group of metals (Li, Na, K, Rb, Cs, and Fr), the halogen group of elements (F, Cl, Br, I, and At), or combination thereof.

The system also teaches treating the anode such that the film is a solid solution layer. Such a layer includes oxygen, carbon, nitrogen, hydrogen, or a combination thereof.

The system additionally teaches an anode film containing a titanium suboxide, a titanium subcarbide, a titanium subnitride, a titanium subhydride, or a combination thereof. Such a layer may be as much as 10 centimeters in thickness.

The method further teaches forming the anode film by anodizing the anode.

These and other objects and advantages of the present invention will become readily apparent to those skilled in this art from the following description wherein there is shown and described a preferred embodiment of this invention, simply by way of illustration of one of the best modes suited for to carry out the invention. As it will be realized, the invention is capable of other different embodiments and its several details are capable of modifications in various obvious aspects all without departing from the spirit of the present invention. Accordingly, the drawing and descriptions will be regarded as illustrative in nature and not as restrictive.

DETAILED DESCRIPTION OF THE DRAWINGS

The present invention relates generally to the fabrication of structures to form anodes using titanium or a titanium alloy. Titanium or titanium alloy anodes advantageously include at least one of titanium oxide, titanium suboxide, titanium nitride, titanium subnitride, titanium carbide, titanium subcarbide, titanium hydride, titanium subhydride, titanium solid solution of carbon, titanium solid solution of hydrogen, titanium solid solution of nitrogen, titanium solid solution of oxygen, and inorganic materials containing titanium in the form of bulk, thin film, precipitates and a combination thereof. Moreover, the anode materials advantageously include an amorphous or crystalline structure, or combination thereof. Titanium or titanium alloy anodes are in any suitable form, including, but not limited to, a sheet, a wire, an individual powder, a powder aggregate, and a thin film. Such anodes are formed by sputtering, electrochemical reaction, ion-implantation, chemical reaction, heat treatment, sintering, oxidation, reduction, carburization, decarburization, nitriding, denitriding, hydrogenization, dehydrogenization, chemical vapor deposition, physical vapor deposition, evaporation, or combination of the foregoing. Thus, an anode in accordance with principles of the present invention is advantageously made of titanium, a titanium alloy, or combination thereof and is suitably used with dielectric and insulated films in capacitors, for example.

Further, a titanium or titanium alloy anode in accordance with principles of the present invention is advantageously heavily deformed on a surface with a thickness of up to 1 millimeter (mm) and/or in bulk with a defect density of 107to 1030/centimeter2(cm2). The objective of such a surface and/or bulk modification is to enhance migration of titanium ions at the interface between an anode film and a titanium or a titanium alloy. This promotes stoichiometric anodic oxide growth by well-balanced titanium ionic transport. Such a titanium or titanium alloy anode is advantageously formed by compression, tension, shot peening, noble gas (e.g., He, Ne, Ar, Kr, Xe and Rn) bombard and impingement, or any mechanical polishing and buffing using rubbing and scratching.

Referring toFIG. 1, a diagram illustrating an exemplary process10of shot peening to heavily deform a surface of a titanium or titanium alloy anode is shown. Process10begins by cleaning a sample12of titanium or a titanium alloy with acetone in an ultrasonic cleaner for approximately thirty (30) minutes and with a mild acid, such as, for example, a 10% HNO3solution. Once sample12is clean, it is loaded in shot peening system14. Shot peening system14comprises a compression gun16. In use, compression gun16propels hard material balls, such as a ceramic or sand, against a surface18of sample12, as generally indicated at reference numeral20. Such propelling of a ceramic or sand against surface18of sample12“shot peens” surface18, deforming the titanium or titanium alloy heavily. Such a deformation enhances the migration of titanium ions at the interface between surface18and an anode film (not shown).

A titanium or titanium alloy anode also advantageously has an amorphous, a random and/or a distorted structure, or a combination thereof, and will supply a higher density of active sites at which to nucleate titanium oxide because of its comparatively unstable nature relative to a crystalline structure. The kinetics of amorphous titanium anodization are also enhanced relative to a crystalline structure. Such a titanium or titanium alloy anode is suitably made by quenching fully or partially molten titanium or titanium alloy to any temperature by any cooling rate.

Referring now toFIG. 2, a diagram illustrating an exemplary process22of quenching a titanium or titanium alloy anode in accordance with principles of the present invention is shown. Process22begins by cleaning sample26of titanium or titanium alloy with acetone in an ultrasonic cleaner for approximately 30 minutes and with mild acid, such as a 10% HNO3solution. Once sample26is clean, sample26is placed in vacuum furnace28of quenching system24and heated to a temperature between approximately 1200 degrees Celsius (° C.) and approximately 2000° C. in a vacuum between approximately 10−6and approximately 10−11Torr. Sample26is then quenched, such as by dropping sample26into cooling unit30of quenching system24, as indicated by arrow31. Cooling unit30advantageously uses liquid nitrogen, dry ice or ice for cooling sample26. Such a process22facilitates the production of a titanium or titanium alloy anode, e.g., sample26, with an amorphous structure that supplies a higher density of active sites at which to nucleate titanium oxide relative to a crystalline structure.

It will be appreciated that, in the alternative, a titanium or titanium alloy anode having an amorphous structure is also suitably attained by physical vapor deposition (PVD), chemical vapor deposition (CVD), electroplating, an electrochemical process, sputtering, thermal evaporation, electron-beam deposition, or laser-pulse deposition, and their modification processes. Further, the substrate for such a titanium or titanium alloy anode may include any other material as well. The substrate temperature is suitably controlled between approximately −273° C. and approximately 300° C.

A titanium or titanium alloy anode is also suitably deposited fully or partially by calcium, calcium oxide, manganese, manganese oxide, magnesium, magnesium oxide, or a combination thereof. Calcium, calcium oxide, magnesium and magnesium oxide readily dissolve in distilled water or a mild acid such that the resulting structure, e.g., anode, is not compromised by their presence. Further, deposition will prevent the incorporation of anion impurities, such as PO43−and SO42−, etc., and thereby control the migration of oxygen ions in the early stages of anodization. If manganese is present during the anodizing process, manganese dioxide is formed preferentially to titanium dioxide. The resulting interfacial stresses help the migration of titanium ions through the oxide, enhancing the kinetics of oxide formation. Manganese controls the concentration of oxygen ions during anodization, thereby regulating the amount of oxygen migrating through the oxide layer. Manganese also forms manganese dioxide, the benefits of which are a simultaneous formation of an anodic film and a solid electrolyte of MnO2. In a deposition of manganese dioxide (hexagonal system), oxygen is easily diffused in a direction parallel to a close packed plane. This material is advantageously used to moderate or enhance oxygen migration during anodization. Additionally, and as previously mentioned, the interfacial stresses help the migration of titanium ions and promote oxide formation. A beneficial effect of manganese dioxide is realized in the development of a thin solid electrolyte.

The deposition of the foregoing materials on an anode are advantageously performed by physical vapor deposition (PVD), chemical vapor deposition (CVD), electroplating, electrochemical process, sputtering, thermal evaporation, electron-beam deposition, or laser-pulse deposition, and their modification process.

Referring now toFIG. 3, a diagram illustrating an exemplary deposition process32using calcium or magnesium, and performed in a vacuum using a thermal evaporation system34is shown. Thermal evaporation system34generally comprises two power supplies36,38and two tungsten filaments40,42, respectively electrically coupled thereto. Thermal evaporation system34further includes a vacuum system44. Vacuum system44comprises a vacuum chamber46and a pump48coupled to the vacuum chamber46, and used to evacuate the vacuum chamber46. Tungsten filaments40,42are located inside vacuum chamber46.

Vacuum chamber46suitably houses an evaporating material50, e.g., calcium or magnesium, and a sample52of titanium or titanium alloy. Power supplies36,38are used to energize tungsten filaments40,42that, in turn, heat sample52and evaporating material50, respectively.

Deposition process32begins by cleaning sample52with acetone in an ultrasonic cleaner for approximately 30 minutes and with mild acid, such as a 10% HNO3solution. Sample52and evaporating material50are then placed in vacuum chamber46and pump48is energized to lower the vacuum in vacuum chamber46down to between approximately 10−6and approximately 10−11Torr. Simultaneously, tungsten filaments40,42are energized using power supplies36,38, respectively, and sample52and evaporating material50are heated to a temperature between approximately 500° C. and approximately 1000° C. Thus, deposition process32forms a titanium or titanium alloy anode, such as illustrated by sample52.

A titanium or titanium alloy anode also advantageously has a surface roughness with a height from approximately 1 nanometer (nm) to approximately 0.1 millimeter (mm), and a width from approximately 1 nm to approximately 0.1 mm, in any suitable shape. Such a surface roughness advantageously increases the surface area and surface energy, promotes the nucleation of anodic titanium oxide, and forms layered growth, as opposed to island growth. Such a titanium or titanium alloy anode is realized through mechanical polishing, wet chemical etching, or dry etching using a gas containing the halogen group of elements (e.g., F, Cl, Br, I and At) and/or a mixture of oxygen and nitrogen. Etching and polishing are also advantageously tailored to result in the preferred exposure of close packed planes, which also has the aforementioned effects in terms of forming titanium oxide.

Referring toFIG. 4, a diagram illustrating an exemplary plasma-based dry etching process54is shown. Process54begins by clean a sample56of titanium or titanium alloy with acetone in an ultrasonic cleaner for approximately 30 minutes and with mild acid, such as a 10% HNO3solution and placing sample56onto a cathode60in etching chamber58. Etching chamber56is then evacuated down to approximately 10−6Torr using a pump61coupled thereto.

Next a radio frequency (RF) voltage is applied across cathode60and an anode or, in this example, chamber58using RF power supply68. It will be appreciated that chamber58is also advantageously grounded as shown at reference numeral62to prevent electric shock. A gas64containing argon (Ar) or nitrogen (N2) and chlorine (Cl2) is then introduced into chamber58and plasma radicals and ions impinge on sample56, as indicated by arrows66, etching the sample. Such an etching process54produces a titanium or titanium alloy anode, e.g., sample56, having a surface roughness with a height from approximately 1 nanometer (nm) to approximately 0.1 millimeter (mm), and a width from approximately 1 nm to approximately 0.1 mm in any shape.

A titanium or titanium alloy anode also advantageously has a surface parallel to a close packed plane, a surface up to 40 degrees off a close packed plane, and/or combination thereof. Such a titanium or titanium alloy anode is made by physical vapor deposition (PVD), chemical vapor deposition (CVD), electroplating, electrochemical process, sputtering, thermal evaporation, electron-beam deposition, or laser-pulse deposition, and their modification process. Further, the anode comprises a substrate having an amorphous and/or a crystalline structure, and may also include other materials as well. The substrate is preferably produced at a temperature ranging between approximately −273° C. and 300° C.

Referring toFIG. 5, an exemplary sputtering process70for depositing titanium is shown. Sputtering system71comprises a chamber72that is evacuated by a pump74. A sputtering gas, such as argon (Ar),76is introduced into chamber72to facilitate argon plasma78. Sputtering system71further comprises an RF power supply80electrically coupled to a titanium sputtering target82and a substrate holder84, both of which are housed in chamber72, through ground86.

In use, a substrate90and titanium88is placed on substrate holder84. Thus, sputtering of titanium is achieved in argon (Ar) plasma78. Argon ions with an energy of approximately 1 to 3 KeV bombard titanium sputtering target82and physically dislodge titanium, thereby depositing titanium88on substrate90. Thus, a titanium or titanium alloy anode having a surface parallel to a close packed plane, a surface up to 40 degrees off a close packed plane, and/or combination thereof is produced.

A titanium or titanium alloy anode is also suitably treated and modified by a solid, a liquid, and/or a vapor of hydrogen (H), the alkali group of metals (e.g., Li, Na, K, Rb, Cs, and Fr), the halogen group of elements (F, Cl, Br, I, and At), the IVb group of metals (Ti, Zr, Hf and Rf), carbon, nitrogen, oxygen, hydrogen, iron, sulfur, aluminum, silicon, copper, titanium, niobium, tantalum, yttrium, or a combination thereof in the state of an individual element, a solid solution, a complex and/or an alloy. Such a titanium or titanium alloy anode is preferably produced at any temperature below approximately 1,700° C.

Referring toFIG. 6, a diagram illustrating an exemplary treatment process92using hydrogen (H), the alkali group of metals (e.g., Li, Na, K, Rb, Cs, and Fr), the halogen group of elements (F, Cl, Br, I, and At), or a combination thereof is shown. Process92is conducted in a vacuum chamber94into which a sample98of titanium or titanium alloy and evaporating material100using hydrogen (H), the alkali group of metals (e.g., Li, Na, K, Rb, Cs, and Fr), the halogen group of elements (F, Cl, Br, I, and At), or a combination thereof are placed.

Vacuum chamber94is suitably evacuated by a pump96to a pressure between approximately 10−6and approximately 10−11Torr. Power supplies102,104energize tungsten filaments106,108, respectively electrically coupled thereto. Tungsten filaments106,108, are located proximate sample98and evaporating material100, respectively, in vacuum chamber94. Tungsten filaments106,108thereby simultaneously heat sample98and evaporating material100to a temperature between approximately 500° C. and approximately 1700° C. Thus, a titanium or titanium alloy anode, e.g., sample98, is treated and modified by hydrogen (H), the alkali group of metals (e.g., Li, Na, K, Rb, Cs, and Fr), the halogen group of elements (F, Cl, Br, I, and At), or a combination thereof.

A titanium or titanium alloy anode also advantageously has a solid solution layer including oxygen, carbon, nitrogen, hydrogen, or a combination thereof. The solubility of oxygen, carbon, nitrogen and hydrogen is 50, 10, 40, and 75 atomic percent, respectively. The thickness of the solid solution layer is suitably as much as 10 centimeters (cm). Such a titanium or titanium alloy anode is produced at any temperature below approximately 1,700° C. and in any environment including oxygen, carbon, nitrogen, hydrogen, or a combination thereof. The pressure of the oxygen, carbon, nitrogen, hydrogen, or a combination thereof is preferably below 760 Torr.

Referring toFIG. 7, a diagram illustrating an exemplary process110of treating titanium or a titanium alloy using a solution in a vacuum system112to form a titanium or titanium alloy anode is shown. Vacuum system112generally comprises a vacuum chamber114and a pump116coupled thereto, and used to evacuate vacuum chamber114. Vacuum system112further comprises a power supply118and a tungsten filament120electrically coupled thereto, and located within vacuum chamber114. Power supply118and tungsten filament120are used to heat a sample122of titanium or titanium alloy.

Process110begins by cleaning sample122with acetone in an ultrasonic cleaner for approximately 30 minutes and with mild acid, such a 10% HNO3solution. Sample122is then loaded into vacuum chamber114and vacuum chamber114is evacuated down to a vacuum between approximately 750 and approximately 10−11Torr. Next, sample122is heated to a temperature between approximately 700° C. and approximately 1400° C. for approximately 120 minutes using power supply118and tungsten filament120. Thus, process110produces a titanium or titanium alloy anode, e.g., sample122, having a solid solution layer including oxygen, carbon, nitrogen, hydrogen, or a combination thereof.

A titanium or titanium alloy anode also advantageously has a titanium suboxide, a titanium subcarbide, a titanium subnitride or a titanium subhydride layer, or a combination thereof. The chemical compositions of a titanium suboxide, a titanium subcarbide, a titanium subnitride, and a titanium subhydride are TiOx(where x<2), TiCx(where x<1), TiNx(where x<1) and TiHx(where x<2), respectively. The thickness of each layer or a combination thereof is suitably as much as 10 cm. Such a titanium or titanium alloy anode is produced at any temperature below approximately 1,700° C. and in any environment including oxygen, carbon, nitrogen, hydrogen, or a combination thereof. The pressure of the oxygen, carbon, nitrogen, hydrogen, or a combination thereof is preferably below 760 Torr.

Referring now toFIG. 8, an exemplary process124of treating a titanium or titanium alloy using a vacuum system126such that the titanium or titanium alloy has a titanium suboxide layer. Vacuum system126generally comprises a vacuum chamber128and a pump130coupled thereto, and used to evacuate vacuum chamber128. Vacuum system126further comprises a power supply132and a tungsten filament134electrically coupled thereto, and located within vacuum chamber128. Power supply132and tungsten filament134are used to heat a sample136of titanium or titanium alloy.

Process124begins by cleaning sample136with acetone in an ultrasonic cleaner for approximately 30 minutes and with mild acid, such a 10% HNO3solution. Sample136is then loaded into vacuum chamber128and vacuum chamber128is evacuated down to a vacuum between approximately 750 and approximately 10−8Torr. Next, sample136is heated to a temperature between approximately 700° C. and approximately 1400° C. for approximately 60 minutes using power supply132and tungsten filament130. Thus, process124produces a titanium or titanium alloy anode, e.g., sample136, having a titanium suboxide layer.

A titanium or titanium alloy anode also advantageously anodized at any temperature between approximately −273° C. and approximately of 300° C. If the electrochemical reaction of cell including titanium or titanium alloy is exposed to decreasing temperature, the titanium and oxygen ionic transport will be balanced to grow the stoichiometric oxide during anodization. Such an electrochemical reaction is applicable to the anodization of a titanium or titanium alloy anode.

Referring toFIG. 9, an exemplary process140of anodizing a titanium or titanium alloy anode is shown. Process140begins by cleaning a sample142of titanium or titanium alloy with acetone in an ultrasonic cleaner for approximately 30 minutes and with mild acid, such as a 10% HNO3solution. Sample142is then placed in an electrolyte, such as a 1% H3PO4solution, and sample142and electrolyte144are placed into a cooling unit146. Cooling unit146advantageously uses dry ice or ice for anodizing. Thus, process140anodizes a titanium or titanium alloy anode, e.g., sample142.

Alternatively, and once cleaned, sample142is placed in a liquid nitrogen reservoir for approximately 2 hours. Sample142is then taken out of the reservoir and placed immediately into an electrolyte. Such an alternative also anodizes a titanium or titanium alloy anode, e.g., sample142.

The processes described in conjunction withFIGS. 6–8may also be used with systems150,152,154shown inFIGS. 10–12, respectively, and wherein like numbers denote like parts. Generally systems150,152,154ofFIGS. 10–12comprise some combination of a sample container156, a protection container158, a heating apparatus160, a cooling apparatus162, and a vacuum system164. Sample container156is used to house a titanium powder aggregate166that, through a process, e.g., a combination of heating, cooling, pressurizing, etc., is formed into anode. Protection container158is used to contain sample container156in the event that sample container156fails during a process, such as, for example, failure due to pressures experienced during a process. Both sample container156and protection container158include a caps168and170, respectively. Any closing or sealing method for caps168,170may be used, such as, for example welding, gasketing like copper o-rings172, fastening, e.g., bolts174or combination thereof. Protection container158thus provides some measure of user protection.

More specifically, system150ofFIG. 10comprises a sample container156having an inlet176and outlet178. Coupled to inlet176is source of gas in the form of a gas tank180. Coupled to outlet178is a vacuum system164. Valve182control the ingress of gas into sample container156while valve184controls the application of a vacuum to sample container156. Likewise, system152ofFIG. 11comprises a sample container156housing a titanium powder aggregate166. System152further comprises protection container158that houses sample container156. Similarly, system154ofFIG. 12comprises a heating system160for directly, or solely, heating titanium powder aggregate166and a cooling system162for cooling sample container156. A heating system160is also suitably used to heat sample container156containing a titanium powder aggregate166, as shown inFIGS. 10 and 11.

The dimensions of sample container156and protection container158are not restricted or limited in size. Sample and protection containers156,158of any size having any wall thickness, volume, diameter, length, height, shape, etc. may be suitably used. Moreover, sample and protection containers156,158are suitable constructed of solid materials including, but not limited to, ceramics, metals, composites, semiconductors, glass, or a combination thereof, and have the ability to prevent the penetration of air and/or gas into or out of the containers. Some suitable examples are stainless steel, cast iron, nickel alloy, and cobalt alloy.

Additional materials, such as those discussed in conjunction withFIG. 6, are inserted into sample container156along with titanium powder aggregate166in processing. The additional materials are suitably in a liquid, solid or vapor state, examples of which follow: solid or liquid materials containing carbon, hydrogen, nitrogen, oxygen, or combination thereof, gases containing carbon, hydrogen, nitrogen, oxygen, or combination thereof, and periodic elements of group VII (F, Cl, Br, I and At), tantalum oxide, tantalum alloy, niobium oxide, niobium alloy, sodium oxide, sodium chloride, silicon, silicon oxide, silicon alloy, iron, iron alloy, molybdenum, molybdenum alloy, titanium, titanium alloy, titanium carbide, titanium nitride, titanium oxide, titanium hydride, aluminum, aluminum oxide, aluminum alloy, and combinations thereof.

Still referring toFIGS. 10–12, additional processes and process limitations will now be described. For example, sample container156is evacuated, such as by vacuum system164to a vacuum below 760 Torr, preferably 10−6Torr, by closing valve182and opening valve184. Sample and protection containers156,158, along with titanium powder aggregate166, are heated to a temperature above 300° C., preferably approximately 900 to 1000° C., for a preferred period of approximately 7 hours. Alternatively, sample and protection containers156,158and titanium powder aggregate166are heated to a temperature above 300° C., preferably approximately 900° C., for a preferred period of 30 seconds, and a vacuum below approximately 760 Torr, preferably approximately 10−11Torr is applied. Sample and protection containers156,158and titanium powder aggregate166are heated to a temperature above approximately 300° C., preferably approximately 1000° C., for a preferred period of approximately 5 hours.

Alternatively still, a process introduces one of the abovementioned gases into a sample container156. Such a gas is introduced simultaneously while heating sample and protection containers156,158and titanium powder aggregate166to a temperature above approximately 300° C., preferably 1000° C., for preferably approximately 1 minute. A vacuum below approximately 760 Torr, preferably 10−11Torr, is applied and heating continues at a temperature above approximately 300° C., preferably 1200° C. It will be appreciated by those of ordinary skill in the art that neither the flow velocity or the amount of the gas is, in any way, restricted or limited.

Alternatively still, a process introduces one of the abovementioned gases into a sample container156, and pressurizes sample container156to a pressure above approximately 760 Torr, preferably approximately 900 Torr. Heating for a preferred period of approximately 10 minutes to a temperature above approximately 300° C., preferably approximately 350° C., is applied. The process reheats at a temperature above approximately 300° C., preferably approximately 600° C., for a preferred period of approximately 30 minutes. Cooling to a temperature below approximately 1800° C., preferably approximately 100° C., is applied along with pressurization below approximately 760 Torr, preferably approximately 10−11Torr. A second reheating at a temperature above approximately 300° C., preferably approximately 1200° C., for a preferred period of preferably 3 hours is applied.

Those of ordinary skill in the art will appreciate that many other processes are possible to achieve a desired pretreatment of titanium powder aggregate166for use as an anode. Such processes suitably include the method steps of: loading titanium powder aggregate166to be processed into sample container156, sealing sample container156using an o-ring172, cap168, valves182,184, pressurizing sample container156to a pressure of approximately 760 Torr, pressurizing sample container156to a pressure below approximately 760 Torr for some period of time, pressurizing sample container156to a pressure above approximately 760 Torr using one of the aforementioned gases, placing at least one of the abovementioned additive solid and liquid materials in sample container156, introducing at least one of the aforementioned gases into sample container156, placing sample container156housing titanium powder aggregate166into protection container158and sealing protection container158by at least one of welding, capping, and valving, pressurizing the volume between sample container156and protection container158to a pressure below approximately 760 Torr, heating sample container156and titanium powder aggregate166at a temperature above approximately 300° C. for some suitable period of time, heating titanium powder aggregate166at a temperature above approximately 300° C. for some suitable period of time, and heating sample container156at a temperature above approximately 300° C. for some suitable period of time.

Method steps advantageously further include: heating sample container156and titanium powder aggregate166in a sequence of temperatures above approximately 300° C. for some period of time, e.g., 500° C.→800° C.→900° C.→1200° C.→1400° C., heating titanium powder aggregate166in a sequence of temperatures above 300° C. for some period of time, and heating sample container156in a sequence of temperatures above 300° C. for some period of time. Still further, method steps advantageously include: cooling sample container156and titanium powder aggregate166to a temperature below approximately 1800° C., cooling titanium powder aggregate166to a temperature below approximately 800° C., cooling sample container156to a temperature below approximately 1800° C., and cooling at least one of sample container156and titanium powder aggregate166, and a combination thereof in a sequence of temperatures below 1800° C. for some period of time, e.g., 900° C.→850° C.→800° C. →600° C.→20° C.→10° C.→40° C.

Thus, processes described herein generally comprise heating, cooling and the application of a vacuum. Those of ordinary skill in the art will appreciate that any suitable combination or sequence of the foregoing method steps can be used. Moreover, any cooling and heating rate can be used. Further, the period of time for heating is anywhere from approximately 1 minute to as much as 5,000 hours, depending on the temperature, the volume and surface area of titanium powder aggregate166, the system150,152,154used, etc. Likewise, the temperature and time can change from one process to another.

Earlier capacitors using titanium or titanium alloy and incorporating anode films are typically characterized by a high density of inherent defects, such as microvoids, microcrystallites, oxygen vacancy, suboxides, and variation of nonstoichiometry. These inherit defects are manifested in unstable dielectrics and unacceptable electric performance including excessive leakage currents and large variations in capacitance values. In contrast, the aforementioned processes produce a near ideal capacitor-quality anode film. This homogeneous and defect-free anode film when used in a capacitor allows the capacitor to exhibit excellent electrical characteristics including, high capacitance, long-term stability, high energy density and/or high power density.

A thickness of an anode film is determined theoretically by the current density used to anodize and the time required to reach the rated voltage in accordance with Faraday's law:
d=(∫Idt)M/nAFρ
where d is the thickness, I is the current, t is the time, M is molecular weight of the anode oxide, n is the number of electron exchanged to form one oxide molecule, A is anodized surface area, F is 96,490C/mol, and ρ is the density of the oxide. In earlier capacitors using titanium or titanium alloy and incorporating anode films, M, n and ρ varied and were unpredictable because of the defective structure having a different stocichiometry of the oxide; and thus, the thickness of the oxide was not consistent and uniform. In contrast, the subject processes produce an anode film with a generally consistent, reproducible and uniform thickness due to M, n and ρ being relatively constant. Moreover, the thickness per voltage will be approximately 1–10 nm, depending on the particular process used. Further, the forgoing processes generally provide relatively low leakage currents and a well-controlled capacitance values.

Ideal dielectric materials exhibit a linear relationship between capacitance and voltage having a negative slope, that is described by the equation:
CV=constant,
where C is capacitance and V is the anodizing voltage. Earlier capacitors using titanium typically failed to preserve this relationship. In contrast, the aforementioned processes generally preserve this relationship. Furthermore, the capacitance of capacitors made using the aforementioned processes at de-rated voltages generally measure the same at that of the anodizing voltage. This factor is often used to judge a dielectric film for use in capacitors. Thus, capacitors using dielectric films produced using the aforementioned processes typically exhibit a higher capacitance voltage per gram (CV/g), e.g., up to 10 times more than that of tantalum and niobium capacitors.

Leakage current is another parameter used to judge capacitors and is of critical concern as failure mechanism during operation. Earlier capacitors using titanium or titanium alloy and incorporating anode films typically exhibited leakage current densities in the range of 11–4,000 μA/cm2based on anodizing voltages of 10–200 volts. Moreover, the leakage current density at de-rated voltages was still high, so failures readily occurred. In contrast, capacitors made using the aforementioned processes typically exhibit leakage current densities ranging approximately from 1–100 μA/cm2based on based on anodizing voltages of 10–200 volts and the leakage current density at de-rated voltages is typically below 1 nA/cm2.

The long-term stability for capacitors using titanium or titanium alloy and incorporating anode films is practically the fault current. The fault current could pass through the bulk of the dielectric, through defects in the dielectric, or in route by passing the dielectric and bridging between the anode and the cathode. Although leakage current paths may not be the paths taken by a fault current, the ratio of the leakage current at working voltage to the leakage current at an anodizing voltage should be lower than a value y in the following equation:
y=0.982x+0.018,
where y represents the dropping ratio of two leakage currents, and x is a de-rated voltage ratio. In prior art capacitors using titanium or titanium alloy and incorporating anode films, the leakage current ratio typically exceed the value y, disqualifying an anode film from use in such a capacitor. In contrast, the leakage current ratio typically exhibit by films using the aforementioned process is generally much less than the value y due to ideal nature and stability exhibited by the film.

Another advantage of the foregoing processes is to prevent or decrease the incorporation of anion impurities such as, for example, PO43−and SO42−, and water from the electrolyte solution into a film at the film/solution interface. Incorporated anions markedly reduce the dielectric constant of a film and increase the electric field required for a given ionic current density. Similarly, a high content of hydrating water or an OH−bridge in the film also results in a relatively high dielectric constant and low donor density. Thus, the foregoing processes have an advantage by providing an ability to control anion impurities and water to achieve a desired specification.

Further, the foregoing processes produce an anode film having an energy density on the order of 60 J/g, which is generally 10 times higher than that of an anode film of tantalum pentoxide (6 J/g). In addition, an anode film produced by the foregoing processes does not generally need a self-healing layer of MnO2or polymer commonly used in package capacitors, such as multilayer ceramic capacitors (MLCC).

In an ideal dielectric film, the capacitance is inversely proportional to the thickness of the dielectric film at the formation voltage and at de-rated voltages. Again, the aforementioned processes produce a near ideal capacitor-quality anode film. Such a film is suitably of titanium oxide. Table 1 shows the capacitance of a film of titanium oxide produce using the aforementioned processes at different anodizing and de-rated voltages, and in comparison to anodized tantalum oxide.

TABLE 1Capacitance of anodized titanium oxide at different anodizing andde-rated voltages including the capacitance of anodized tantalumoxide for comparison.AnodizingDeratedCapacitance ofvoltagevoltageanodized titaniumCapacitance of anodized(V)(V)oxide (μF/cm2)tantalum oxide (μF/cm2)1051.191.181.2161.181.17N/A71.181.19N/A81.181.18N/A91.171.181.192050.620.600.6260.600.61N/A70.610.59N/A80.590.590.6190.620.580.603050.380.38N/A60.390.38N/A70.370.390.4080.400.380.4190.390.400.41
Each value in Table 1 represents an individual sample, yet each sample uses the same process or pretreatment. The samples demonstrate reproducibility, reliability, and consistency of the results shown. Again, the capacitances of anodized tantalum oxide are included in Table 1 for purposes of comparison. It will be appreciated that, as shown in Table 2, the capacitance and energy density of anodized titanium oxide is dependent on the pretreatment process.

Table 2 shows capacitance values of 0.6 and 2.9 μF/cm2for the anodized titanium oxide at an anodizing voltage of 20 Volts in a dilute aqueous phosphoric acid, and in comparison with a capacitance value of 0.6 μF/cm2for anodized tantalum oxide at the same anodizing voltage and in the same electrolyte.

TABLE 2Dependence of the capacitance and energy density of titanium oxideon the pretreatment process including the capacitance and energydensity of anodized tantalum oxide for comparison.EnergyEnergy densityCapacitancedensityCapacitanceof anodizedof anodizedof anodizedAnodizingof anodizedtitaniumtantalumtantalumvoltagetitanium oxideoxide at 5 Voxideoxide at 5 V(V)(μF/cm2)(J/cm2)(μF/cm2)(J/cm2)200.67.500.67.50202.936.25
Based on the anodizing voltage and the demonstrated capacitance, the energy density of the anodized titanium oxide is calculated to be 7.50 and 36.25 J/cm2at the rated 5 Volts, and as shown in Table 2. It will be appreciated that the energy density of anodized tantalum oxide at the same rated voltage is 7.50 J/cm2. Thus, in comparison, the capacitance and energy density of the anodized titanium oxide is same or as much as 4.83 times greater than that of anodized tantalum oxide, depending on the process.

Table 3 shows the leakage current of the anodized titanium oxide, and was obtained at values of 0.25 and 0.80 nA/μFV at a de-rated voltage of 5 Volts (formation voltage of 20 V) after 3 minutes in a dilute aqueous phosphoric acid.

TABLE 3Leakage current of the anodized titanium oxide including theleakage current of anodized tantalum oxide for comparison.Leakage current ofLeakage current ofanodized titaniumanodized tantalumAnodizingDeratedoxideoxidevoltagevoltageIn unit ofIn unit ofIn unit ofIn unit of(V)(V)nA/μFVnA/cm2nA/μFVnA/cm22050.253.61.675.02050.802.4
As tabulated, both of these leakage currents are below a practical value of 10 nA/μFV and, therefore, are suitable as a dielectric film for a capacitor. As also tabulated, anodized tantalum oxide exhibits a value of 1.67 nA/μFV for at the same rated voltage, formation voltage, time and electrolyte. In terms of measures per unit area, 0.25, 0.80 and 1.67 nA/μFV correspond to 3.6, 2.4 and 5.0 nA/cm2, respectively. The tabulated data demonstrates that anodized titanium oxide has better insulating properties than anodized tantalum oxide. Further, it was found that even after a constant 5 V was applied to the anodized titanium oxide in a dilute aqueous phosphoric acid for 18 hours, the low leakage current of 2.4 nA/cm2was maintained. Thus, the film does not demonstrated a typical leakage behavior commonly associated with anodized titanium oxide; that is, the leakage current decreases at the beginning, then increasing between approximately two and twenty minutes, and never decrease down to 10 nA/μFV or less.

By virtue of the foregoing, inconsistent capacitance and the high and variable leakage current behaviors associated with the use of titanium and titanium alloys for anodes are resolved. Moreover, leakage current and capacitance in the processing or manufacture of capacitors using titanium and titanium alloy anodes is controlled. Further, fabrication techniques for anodes using titanium or a titanium alloy are provided.