Patent Application
20240337037
This disclosure relates generally to metal or metal alloy formation from metal oxide precursors. More particularly, embodiments of the disclosure relate to methods of forming the metal or metal alloy by electrochemical reduction and to the metal or metal alloy exhibiting increased porosity and purity.
Stoichiometric metal oxides, either sintered or un-sintered, have been used as starting materials (e.g., precursors) for producing metals or metal alloys by an electrochemical reduction process. Sintered oxide bodies are fabricated from the stoichiometric metal oxides by a powder metallurgical process that involves mixing, pelletization, and air-sintering. However, disadvantages of these processes include low current efficiencies and high levels of impurities, such as residual oxygen. Therefore, the manufacture of high purity metals and alloys cannot be achieved by these processes alone.
Various embodiments of the disclosure provide a method of forming a metal material. The method comprises exposing one or more metal oxide materials to one or more of a reducing agent and a reducing atmosphere to form one or more non-stoichiometric metal oxide materials. The one or more non-stoichiometric metal oxide materials are electrochemically reduced to a metal material or a metal alloy.
A system is also provided. The system comprises one or more electrochemical cells comprising a counter electrode, a working electrode in electrical communication with the counter electrode, the working electrode comprising one or more non-stoichiometric metal oxide materials, and an electrolyte comprising a molten salt. The counter electrode and the working electrode are disposed in the electrolyte. A power source is electrically coupled to the counter electrode and the working electrode and is configured to provide a current flow between the counter electrode and the working electrode.
A metal article comprising a porous metal or a porous metal alloy exhibiting an oxygen content of less than or equal to about 1200 parts per million is also provided.
For a detailed understanding of the disclosure, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, wherein:
As used herein, the singular forms following “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise.
As used herein, the term “may” with respect to a material, structure, feature, or method act indicates that such is contemplated for use in implementation of an embodiment of the disclosure, and such term is used in preference to the more restrictive term “is” so as to avoid any implication that other compatible materials, structures, features, and methods usable in combination therewith should or must be excluded.
As used herein, any relational term, such as “first,” “second,” “top,” “bottom,” “upper,” “lower,” “above,” “beneath,” “side,” “upward,” “downward,” etc., is used for clarity and convenience in understanding the disclosure and accompanying drawings, and does not connote or depend on any specific preference or order, except where the context clearly indicates otherwise. For example, these terms may refer to an orientation of elements of any electrochemical cell. Furthermore, these terms may refer to an orientation of elements of any system as illustrated in the drawings.
As used herein, the term “substantially” in reference to a given parameter, property, or condition means and includes to a degree that one skilled in the art would understand that the given parameter, property, or condition is met with a small degree of variance, such as within acceptable manufacturing tolerances. By way of example, depending on the particular parameter, property, or condition that is substantially met, the parameter, property, or condition may be at least 90.0% met, at least 95.0% met, at least 99.0% met, or even at least 99.9% met.
As used herein, the term “about” used in reference to a given parameter is inclusive of the stated value and has the meaning dictated by the context (e.g., it includes the degree of error associated with measurement of the given parameter, as well as variations resulting from manufacturing tolerances, etc.).
A metal or a metal alloy may be electrochemically produced from a metal oxide material (e.g., a metal oxide precursor). The metal or the metal alloy according to embodiments of the disclosure may be produced at a higher purity and at a greater current efficiency than metals or metal alloys produced by conventional techniques. The metal or the metal alloy are collectively referred to herein as a metal material. The metal oxide material from which the metal material is produced may be exposed to process conditions that produce mobile paths in a matrix of the metal oxide material. For instance, the metal oxide material may be combined with a reducing agent and/or a fugitive agent, the metal oxide material may be exposed to a reducing atmosphere, and/or the metal oxide material may be selected to exhibit an open microstructure (e.g., an open crystal structure). The metal oxide material is heated and electrochemically reduced to form the metal material (e.g., the metal, the metal alloy). The metal material may also be produced by applying a controlled voltage/current and/or by gradually or sequentially increasing the voltage/current in a pre-determined manner during the electrochemical reduction of the metal oxide material.
The mobile paths may enable oxygen, in the form of oxide ions (O2−), to diffuse from (e.g., leave) a matrix of the metal oxide material. The process conditions ionize oxygen atoms in the metal oxide material to form the oxide ions, which migrate from (e.g., exit) the matrix of the metal oxide material through the mobile paths, are transported to an electrolyte of an electrochemical cell, and travel to a surface of an anode of the electrochemical cell during the electrochemical reduction. The oxide ions deposit electrons on the anode surface and are then discharged from the anode surface in the form of molecular oxygen. Discharged electrons travel back to the oxide matrix during the electrochemical reduction, oxygen atoms pick up the electrons and become oxygen ions, and the oxygen ions diffuse out of the oxide matrix.
The mobile paths are produced by one or more of adding the reducing agent to the metal oxide material, by adding the fugitive agent to the metal oxide material, by exposing the metal oxide material to the reducing atmosphere, by choosing the appropriate microstructure of the oxide matrix, and by applying a controlled voltage/current during the electrochemical reduction of the metal oxide material. In other words, the mobile paths may be formed by increasing the internal porosity of the metal oxide material and increasing chemical inhomogeneity and/or injecting defects into the metal oxide material. The mobile paths may also be achieved by appropriately selecting the metal oxide material to exhibit a relatively open microstructure (e.g., a relatively open crystal structure). By increasing the mobile paths in the metal oxide material, the oxide ions may more efficiently and quickly exit the matrix, increasing the kinetics of the electrochemical process and, thereby, improving the overall purity of the metal material electrochemically produced from the metal oxide material. In addition to the increased porosity, the pores may be highly interconnected with one another and form paths in the metal oxide material through which the oxide ions are able to diffuse quickly. The paths may extend between surfaces of the metal oxide material. The metal material is formed as the result of oxygen loss from the metal oxide material following the electrochemical reduction. The metal material may exhibit increased porosity relative to the metal oxide material. In comparison, conventional electrochemical techniques of reducing a metal oxide material to a metal material use a stable (e.g., stoichiometric metal oxide material) that exhibits a comparatively low porosity and a relatively closed/dense microstructure. However, many metal oxide materials do not have a stable oxide form that is suitable for use with conventional powder metallurgy techniques. In addition, the stoichiometric metal oxide material used in conventional electrochemical techniques may include tortuous paths of pores that are not line-of-sight paths. Thus, the stoichiometric metal oxide does not exhibit sufficient mobile paths such that oxygen ions must travel through the tortuous diffusion paths. This may necessitate a longer reduction time and may result in a higher than desired residual oxygen content in the metal material.
The metal oxide material may be a group III metal oxide, a group IV metal oxide, a transition metal oxide, a lanthanide oxide, an actinide oxide, or a combination thereof. The metal oxide material may include, but is not limited to, an aluminum oxide (Al2O3), a chromium oxide (Cr2O3), a nickel oxide (NiO), a silicon oxide (SiO2), a germanium oxide (GcO2), a hafnium oxide (HfO2), a vanadium oxide (V2O5, V2O3), an iron oxide (FcO, Fc2O3, Fc3O4), a lanthanum oxide (La2O3), a praseodymium oxide (Pr2O3, Pr6O11), a neodymium oxide (Nd2O3), a cerium oxide (CeO2), a cobalt oxide (CoO, Co2O, Co2O3), a magnesium oxide (MgO), a molybdenum oxide (MoO2), a niobium oxide (e.g., Nb2O5), a platinum oxide, a tantalum oxide (e.g., Ta2O5), a titanium oxide (e.g., TiO2), a tungsten oxide (WO2, WO3), a zinc oxide (ZnO), a zirconium oxide (ZrO2), a samarium oxide (Sm2O3), a europium oxide (EuO, Eu2O3), a gadolinium oxide (Gd2O3), a terbium oxide (TbO, Tb2O3, Tb6O11), a dysprosium oxide (Dy2O3), a holmium oxide (Ho2O3), an erbium oxide (Er2O3), a thulium oxide (Tm2O3), a ytterbium oxide (Yb2O3), a lutetium oxide (Lu2O3), a uranium oxide (UO2), a plutonium oxide (PuO2), a thorium oxide (ThO2) or a combination thereof. The metal oxide material may be a stoichiometric metal oxide material or a non-stoichiometric metal oxide material. A single metal oxide material may be used in the electrochemical reduction to form the metal, while a combination of metal oxide materials (e.g., mixed metal oxide materials) may be used in the electrochemical reduction to form the metal alloy. The metal oxide material may be a primary metal oxide feedstock, such as a metal oxide available from a mining process, or may be obtained from a waste material by a recycling process. Alternatively, the metal oxide material may be a directly mined material, such as ilmenite (FeTiO3).
The metal oxide material may be configured as a metal oxide preform that includes one or more of the metal oxide materials and an organic binder, such as one or more of polyvinyl alcohol, polyethylene glycol, acetone, and isopropyl alcohol. The organic binder may be a solid binder, such as polyvinyl alcohol (PVA), or a liquid binder, such as polyethylene glycol (PEG). The metal oxide material and the organic binder may be combined in relative ratios sufficient for the metal oxide preform to be formed with compaction. By way of example only, the metal oxide material may be present at from about 10% by weight (wt. %) to about 99 wt. %, with the organic binder being present at from about 1 wt. % to about 90 wt. %, such as from about 1 wt. % to about 2 wt. %. If, for example, a solid organic binder is used, the solid binder may be present at from about 1 wt. % to about 2 wt. %. If, for example, a liquid organic binder is used, the liquid binder may be present at from about 5 drops/20 g of metal oxide material to about 10 drops/20 g of metal oxide material. The metal oxide material and the organic binder may be ball-milled for several hours (e.g., about 12 hours) prior to compacting the powder into pellets and or any other geometry (e.g., cylindrical). The metal oxide material and the organic binder may be consolidated (e.g., compacted, pressed) under pressure into the metal oxide preform, such as by using a hydraulic press or other press. The metal oxide material and the organic binder may be compacted for an amount of time sufficient to form the metal oxide preform. The metal oxide preform may, for example, be configured as pellets, cylinders, etc.
To electrochemically produce the metal or metal alloy, the metal oxide material may be heated to a temperature greater than or equal to about 500° C. By way of example only, the metal oxide material may be heated to a temperature of from about 500° C. to about 1500° C., such as from about 550° C. to about 1500° C. or from about 600° C. to about 1500° C.
To form the mobile paths in the metal oxide material (or mixed metal oxide material), a reducing agent may be added to the metal oxide material, such as the stoichiometric metal oxide material, and heated (e.g., sintered) to form pores. The reducing agent may be a hydride compound, such as gallium hydride (GaH3), vanadium hydride (VH2, VH), chromium hydride (CrH2, CrH), titanium hydride (TiH, TiH2, TiH3, TiH4), zirconium hydride (ZrH4), or a combination thereof. A powder of one or more metal oxide materials may be heated with the reducing agent to form the pores and/or non-stoichiometry in the resulting non-stoichiometric metal oxide material or non-stoichiometric mixed metal oxide material. The metal oxide material may be heated to a temperature of from about 950° C. to about 1500° C., such as from about 1000° C. to about 1500° C. or from about 1050° C. to about 1500° C. The reducing agent and the metal oxide material may form the oxygen-deficient oxide (e.g., non-stoichiometric metal oxide material) and one or more gaseous reaction products when heated, which produce the pores in the metal oxide material, from which the metal or metal alloy is produced. The gaseous reaction may produce hydrogen (H2), which, in turn, provides an in situ reducing atmosphere.
Exposing one or more metal oxide materials to one or more of a reducing agent and a reducing atmosphere forms one or more non-stoichiometric metal oxide materials. The reducing agent and the metal oxide material, when heated, produce an oxide of the reducing agent and carbon dioxide (CO2), which is gaseous at a temperature at which the process is conducted. The CO2 gas produces the pores in the metal oxide material. By increasing the porosity, the mobile paths in the matrix of the formed non-stoichiometric metal oxide material may be increased relative to the mobile paths in the initial metal oxide material (i.e., the metal oxide material before exposure to heat and the reducing agent). The increased porosity enables the oxide ions to leave the matrix of the non-stoichiometric metal oxide material when it is later reduced to form the metal or metal alloy. The initial metal oxide material may exhibit a porosity of less than or equal to about 5%, while the non-stoichiometric metal oxide material may exhibit a porosity of greater than or equal to about 10%. Depending on the metal of the metal oxide material, the initial porosity may be from about 1% to about 5%. Depending on the metal of the metal oxide material, the porosity of the reduced metal oxide material (i.e., the non-stoichiometric metal oxide material) may be from about 10% to about 60%, such as from about 10% to about 50%, such as from about 10% to about 40%, from about 10% to about 30%, from about 10% to about 20%, from about 15% to about 50%, from about 20% to about 50%, from about 25% to about 50%, from about 30% to about 50%, from about 35% to about 50%, from about 40% to about 50%, or from about 45% to about 50%. The reduced (non-stoichiometric) metal oxide material may have increased porosity and/or non-stoichiometry (e.g., defects). The pores and/or non-stoichiometry facilitate faster removal of oxygen upon electrochemical polarization. The metal material is then electrochemically formed from the porous non-stoichiometric metal oxide material, as described below.
The metal oxide material may be reacted with the reducing agent for an amount of time sufficient to increase the degree of porosity within the metal oxide material, such as from about 1 hour to about 5 hours. While the exposure to heat and exposure to the reducing agent are described above as being conducted sequentially, the heating and the exposure to the reducing agent of the metal oxide material (e.g., oxide preforms) may be conducted substantially simultaneously.
The metal oxide material may also be combined with a chemical compound (e.g., a so-called “fugitive agent”) that forms a gas when heated with the metal oxide material. The fugitive agent may impart chemical inhomogeneity to the metal oxide material. The fugitive agent may be a metal carbonate or a metal oxalate. For example, the fugitive agent may be one or more of lithium carbonate (Li2CO3), calcium carbonate (CaCO3), calcium oxalate (CaC2O4), and barium oxalate (BaC2O4). For example, calcium carbonate may decompose to calcium oxide and CO2. When utilized in a non-reducing atmosphere (e.g., in an oxygen atmosphere), the CO2 exits the metal oxide material, creating pores that are distinct, randomly distributed, and unconnected. Without wishing to be bound by theory, it is believed that the fugitive agent alone may not form the mobile paths in the metal oxide material, but when the fugitive agent is present with the metal oxide material and heated in a reducing atmosphere, interconnected pores (e.g., the mobile paths) are formed in the metal oxide material. The fugitive agent may be combined with the metal oxide material at any desired amount. In embodiments, the fugitive agent is present at trace quantities, such as from about 1 part per million (ppm) to about 20 ppm (from about 0.0001 weight percent to about 0.002 weight percent) or from about 1 part per million to about 10 ppm (from about 0.0001 weight percent to about 0.001 weight percent). In further embodiments, the fugitive agent may be present at from about 5,000 part per million (ppm) to about 20,000 ppm (from about 0.5 weight percent to about 2 weight percent).
Alternatively, the mobile paths may be formed by exposing the metal oxide material (e.g., the initial metal oxide material) to a reducing atmosphere to convert a stoichiometric metal oxide material to a non-stoichiometric metal oxide material, which is then subjected to the electrochemical process to produce the metal material. The initial metal oxide material may be a stoichiometric metal oxide material represented as MxOy, where x and y are natural numbers between 1 and 8. By way of example only, the metal oxide material may be Ta2O5 or TiO2. The reducing atmosphere may include a reducing gas, such as hydrogen gas (H2), or a combination of hydrogen and an inert gas. The inert gas may include, but is not limited to, argon, nitrogen, helium, or a combination thereof. For instance, the reducing atmosphere may include argon, helium, and hydrogen. The reducing atmosphere may include about 100% hydrogen or the hydrogen may be present in a sufficient amount to remove a portion of oxygen from the oxide when heated (e.g., sintered). A powder of the stoichiometric metal oxide material may be heated to a temperature of from about 950° C. to about 1500° C. in the reducing atmosphere to produce the non-stoichiometric metal oxide material (e.g., the reduced metal oxide material), which is relatively less stable than the stoichiometric metal oxide material. The non-stoichiometric metal oxide material may be represented as MxOy-z, where y is the original oxygen and z is a portion of oxygen removed during the heating. The composition (e.g., the relative ratio of metal atoms to oxygen atoms) of the non-stoichiometric metal oxide material may be further tailored by adjusting an amount of time or a sintering temperature of the process. These conditions may be selected depending on the metal of the metal oxide material. The mobile paths for the oxide ions may be increased by decreasing the stability of the metal oxide material, such as by converting the initial, stoichiometric metal oxide material to the non-stoichiometric metal oxide material and/or injecting defects, such as non-stoichiometry. The metal material is then electrochemically formed from the porous metal oxide material, as described below.
Injection of defects into structurally stable TiO2 to form a non-stoichiometric oxide-deficient TiO2 may reduce the overall reduction time of the non-stoichiometric TiO2 to metal and may result in the formation of a purer titanium metal exhibiting a lower amount of residual oxygen content as compared to conventional methods. A cation-deficient oxide structure or an anion-deficient oxide structure may be produced. However, a cation-deficient structure may result in the loss of titanium metal when the cation-deficient titanium oxide is reduced to titanium metal. In embodiments of the disclosure, an anion-deficient titanium oxide is prepared. The anion-deficient titanium oxide may be created by one or more of removing a portion of oxygen from the oxide (e.g., TiO2) by contacting the oxide (e.g., TiO2) with a reactive gaseous element such as hydrogen, a mixture of hydrogen and argon, or a mixture of hydrogen and helium. In embodiments of the disclosure, contacting the oxide (e.g., TiO2) with hydrogen, a mixture of hydrogen and argon, or a mixture of hydrogen and helium to form the anion-deficient titanium oxide is performed in a static bed reactor or a fluidized bed reactor. The anion-deficient oxide structure may provide advantages including one or more of removal of the non-metallic element at a faster rate from the oxide and preparation of a better quality titanium metal relative to the titanium metal prepared with a cation-deficient titanium oxide.
The reducing gas may be present in the reducing atmosphere at from about 0.5% to about 10.0%, such as from about 1.0% to about 10.0%, from about 1.5% to about 10.0%, from about 2.0% to about 10.0%, from about 2.5% to about 10.0%, from about 3.0% to about 10.0%, from about 3.5% to about 10.0%, from about 4.0% to about 10.0%, from about 4.5% to about 10.0%, from about 5.0% to about 10.0%, from about 0.5% to about 5.0%, from about 1.0% to about 5.0%, from about 1.5% to about 5.0%, from about 2.0% to about 5.0%, from about 2.5% to about 5.0%, from about 3.0% to about 5.0%, from about 0.5% to about 3.0%, from about 1.0% to about 3.0%, from about 1.5% to about 3.0%, from about 2.0% to about 3.0%, or from about 2.5% to about 3.0%. In some embodiments, the reducing gas includes include argon (Ar) and hydrogen gas (H2) and the hydrogen gas is present in the reducing atmosphere at about 5%, at about 3%, or at about 2.9%, with the remainder being argon. The reducing gas may be flowed into a vessel containing the metal oxide material at an appropriate flow rate.
The metal oxide material may be subjected to the reducing atmosphere and heat for an amount of time sufficient to increase the degree of porosity within the metal oxide material, such as from about 1 hour to about 5 hours. Without being bound by any theory, it is believed that the hydrogen gas reacts with oxygen atoms in the metal oxide material, forming water (H2O), which is a vapor at the temperature of the process. The water is removed, which forms the pores in the resulting metal material. While the exposure to heat and the reducing atmosphere are described as being conducted sequentially, the heating and the exposure to the reducing atmosphere may be conducted substantially simultaneously.
By way of example only, titanium dioxide (TiO2) may be heated in a reducing atmosphere that includes hydrogen gas and argon to form a non-stoichiometric titanium oxide, such as TiO2-x, where x is greater than or equal to 0 and less than 1. By way of example only, x may be 0.1, 0.2, 0.3, 0.4, etc. Therefore, the non-stoichiometric titanium oxide electrochemically produced may include, but is not limited to TiO1.2, TiO1.3, TiO1.4, TiO1.5, TiO1.6, TiO1.7, TiO1.8, TiO1.9, etc. For example, if TiO1.6 or TiO1.7 is desired, titanium dioxide (TiO2) may be heated in a reducing atmosphere that includes 100% hydrogen gas. If TiO1.8 or TiO1.9 is desired, titanium dioxide (TiO2) may be heated in a reducing atmosphere that includes a mixture of argon and hydrogen gas.
The desired mobile paths may also be present in a metal oxide material or a combination of metal oxide systems that exhibit less intense packing of atoms, such as an open microstructure (e.g., crystal structure). Depending on the metal oxide material to be used as the precursor, the microstructure of the metal oxide material may, for example, be a simple cubic structure, a body centered cubic (BCC) crystal structure, or a face centered cubic (FCC) crystal structure or a hexagonal structure, or a zincblende and wurtzite structure, or a corundum structure or a rhombohedral structure or a hexagonal structure, which enables the oxygen ions to more easily diffuse out of the matrix. For instance, chromium oxide may be used as the metal oxide material. As an example, a metal oxide material with a simple cubic, a body centered cubic or a face centered cubic crystal structure have about 47.6%, 32%, or 26% void space, respectively, in their lattices and, as a result, the simple cubic structure is more amenable to oxygen removal than a BCC crystal structured metal oxide material. While nickel oxide (cubic lattice) may be used as the single metal oxide material, a mixture of lanthanum oxide (La2O3) with a hexagonal lattice structure and iron oxide (Fe2O3) with a rhombohedral lattice structure may be used as a combination of the mixed metal oxide material.
The electrochemical reduction is then conducted to produce the metal material having the desired mobile paths and increased porosity as described below. The metal oxide material may be electrochemically reduced in a molten salt electrolyte to form the metal material (e.g., the metal or the metal alloy) as described below. The metal oxide material may, for example, be configured as an electrode (e.g., a working electrode) of an electrochemical cell. A voltage/current is applied to the electrochemical cell to electrochemically reduce the metal oxide material.
The resulting metal or the resulting metal alloy (e.g., the resulting metal material) may exhibit a porosity of greater than or equal to about 5% depending on the metal of the metal oxide material, such as from about 5% to about 50%, from about 5% to about 10%, from about 10% to about 40%, from about 10% to about 30%, from about 10% to about 20%, from about 15% to about 50%, from about 20% to about 50%, from about 25% to about 50%, from about 30% to about 50%, from about 35% to about 50%, from about 40% to about 50%, or from about 45% to about 50%. If, for example, the metal or metal alloy includes titanium, the metal material may include a porosity of from about 20% to about 25%. If, for example, the metal or metal alloy includes tantalum, the metal material may include a porosity of from about 40% to about 50%. However, other porosities may be produced. Depending on the experimental conditions, the reduced metal (e.g., tantalum) may even result in the formation of a high surface area powdery product. In comparison, the metal oxide material (e.g., the precursor before the metal material is produced) may exhibit a comparatively higher porosity (in the range 30-60%) depending on the composition of metal oxide material(s).
The electrochemical cell may include a crucible, the working electrode (also referred to as a cathode), a counter electrode (also referred to as an anode), an electrolyte (e.g., the molten salt electrolyte), and an optional reference electrode. The molten salt electrolyte may be an alkali halide salt, an alkaline earth metal halide salt, an alkali oxide, an alkaline earth metal oxide, or a combination thereof. The molten salt electrolyte may include a mixture of metal halides, such as metal chlorides, metal bromides, or metal fluorides. The molten salt electrolyte may include a metal oxide as a functional electrolyte to transport the oxygen from the cathode to the anode. The halides may comprise one or more metal chlorides, one or more metal bromides, and/or one or more metal fluorides. The molten salt electrolyte may, for example, include calcium chloride (CaCl2)), calcium bromide (CaBr2), sodium chloride (NaCl), sodium bromide (NaBr), lithium chloride (LiCl), lithium bromide (LiBr), cesium bromide (CsBr), potassium bromide (KBr), strontium chloride (SrCl2), strontium bromide (SrBr2), or a combination thereof. The functional electrolyte may include calcium oxide and lithium oxide. In some embodiments, the molten salt electrolyte comprises, consists substantially of, or consists of mixtures including chloride-based melt(s) (e.g., calcium-chloride-sodium-chloride (CaCl2)/NaCl), lithium-chloride-lithium-oxide (LiCl/Li2O), calcium-chloride-calcium-oxide (CaCl2)/CaO)), bromide-based melt(s) (e.g., lithium-bromide-lithium-oxide (LiBr/Li2O), calcium-bromide-calcium-oxide (CaBr2/CaO), sodium-bromide-calcium-bromide (NaBr/CaBr2)), or any combination of the foregoing.
In some embodiments, the molten salt electrolyte comprises calcium chloride and calcium oxide (CaCl2—CaO). In some such embodiments, the calcium oxide constitutes between about 1.0 weight percent and about 5.0 weight percent of the molten salt electrolyte, such as between about 1.0 weight percent and about 2.0 weight percent, between about 2.0 weight percent and about 3.0 weight percent, or between about 3.0 weight percent and about 5.0 weight percent of the molten salt electrolyte. The calcium chloride may constitute the remainder of the molten salt electrolyte. In some embodiments, the calcium oxide constitutes about 1.0 weight percent of the molten salt electrolyte. In other embodiments, the calcium oxide constitutes about 5.0 weight percent of the molten salt electrolyte. In some embodiments, when reducing a non-stoichiometric metal oxide comprising tantalum pentoxide, titanium oxide, a lanthanide oxide, an actinide oxide, or combinations thereof, a molten salt electrolyte comprising calcium chloride and calcium oxide is utilized.
The molten salt electrolyte may be maintained at a process temperature such that the molten salt electrolyte remains in a molten state. In other words, the temperature of the molten salt electrolyte may be maintained at or above a melting temperature of the molten salt electrolyte. By way of nonlimiting example, where the molten salt electrolyte comprises lithium chloride and lithium oxide, the temperature of the molten salt electrolyte may be between about 650° C. and about 700° C. Where the molten salt electrolyte comprises calcium chloride and calcium oxide, the temperature of the molten salt electrolyte may be between about 800° C. and about 950° C. Where the molten salt electrolyte comprises sodium chloride and calcium chloride, the temperature thereof may be maintained between about 550° C. and about 950° C. However, the disclosure is not so limited and the temperature of the molten salt electrolyte may be different than those described above.
The working electrode may include the metal oxide material or the mixed metal oxide material immersed in the electrolyte. The working electrode may also form a part of the crucible. The counter electrode may be a carbonaceous material or a non-carbonaceous material. In embodiments, the counter electrode is a non-carbonaceous material. The counter electrode may be formed of and include one or more of graphite (e.g., high density graphite), a platinum group metal (e.g., platinum, osmium, iridium, ruthenium, rhodium, and palladium), an oxygen evolving electrode (tin oxide, SnO2), or another material (e.g., nickel/copper-doped ferrite, NiFc2O4, CuFc2O4). By way of example only, the counter electrode may be formed of and include osmium, ruthenium, rhodium, iridium, palladium, platinum, silver, gold, lithium iridate (Li2IrO3), lithium ruthenate (Li2RuO3), a lithium rhodate (LiRhO2, LiRhO3), a lithium tin oxygen compound (e.g., Li2SnO3), a lithium manganese oxygen compound (e.g., Li2MnO3), calcium ruthenate (CaRuO3), strontium ruthenium ternary compounds (e.g., SrRuO3, Sr2RuO3, Sr2RuO4), CaIrO3, strontium iridate (e.g., SrIrO3, SrIrO4, Sr2IrO4), calcium platinate (CaPtO3), strontium platinate (SrPtO4), magnesium ruthenate (MgRuO4), magnesium iridate (MgIrO4), sodium ruthenate (Na2RuO4), sodium iridate (Na2IrO3), potassium iridate (K2IrO3), or potassium ruthenate (K2RuO4).
The reference electrode may comprise any suitable material and is configured for monitoring a potential in the electrochemical cell. The reference electrode may include nickel, nickel/nickel oxide, glassy carbon, silver/silver chloride, one or more platinum group metals, one or more precious metals (e.g., gold), or combinations thereof. In some embodiments, the reference electrode comprises glassy carbon. The reference electrode may be in electrical communication with the counter electrode and the working electrode and may be configured to assist in monitoring the potential difference between the counter electrode and the working electrode. Accordingly, the reference electrode may be configured to monitor the cell potential of the electrochemical cell. The metal material, platinum group metal, or glassy carbon may be configured as a rod or as a wire.
The electrochemical cell may be housed in an atmosphere-protected environment such as a so-called “glove box,” such as an argon atmosphere glove box, to reduce exposure of sensitive components to moisture and/or oxygen. Alternatively, the electrochemical cell can be operated under a gas-tight arrangement by way of providing a gas inlet and gas outlet arrangement to the electrochemical cell and maintaining a continuous flow of dry argon gas through the electrochemical cell. The crucible is configured to contain the molten salt electrolyte and a basket that contains metal oxide(s) for reducing the metal oxide(s). Each of the working electrode, the counter electrode, and the reference electrode is at least partially disposed in the molten salt electrolyte and in electrochemical contact with the molten salt electrolyte. The molten salt electrolyte may function as a solvent as well as to remove oxygen from the metal oxide(s). When an electrical potential/current is applied between the working electrode and the counter electrode, the metal oxide(s) may be reduced in the electrochemical cell. The applied electrical potential may range from about 1.5 V to about 3.3 V.
In some embodiments, a platinum group metal is used as the counter electrode of the electrochemical cell and a carbon-containing material is used as a reference electrode of the electrochemical cell. In some embodiments, the reference electrode comprises glassy carbon. In other embodiments, the reference electrode comprises nickel, nickel oxide, or a combination thereof. In yet other embodiments, the reference electrode comprises silver/silver chloride.
The desired mobile paths may also be produced by applying a controlled voltage to the electrochemical cell during the electrochemical reduction of the metal oxide material. The voltage may be applied in a stepwise manner for a duration of from about 5 hours to about 10 hours. For example, if the final reduction voltage is about 3.2V, the electrochemical reduction reaction may be initiated at about 2.0V and gradually increased to about 3.2V (this is the case for TiO2). At each voltage (starting from the initial value), the electrochemical reduction reaction is allowed to progress until the cell current approaches a steady value before incrementally raising the voltage. The incremental voltage increase may depend on the metal oxide material or mixed metal oxide material to be formed. The initial voltage selection may depend on the electrolyte system to be used. For a CaCl2/CaBr2—CaO electrolyte system, the initial voltage may be about 2.0V. For LiCl—Li2O the initial voltage may be about 1.5V. The voltage may be applied to the electrochemical cell at a slow rate or stepwise, rather than as a constant voltage that may provide too much energy, overloading the electrochemical cell. The gradual (e.g., relatively slow) or stepwise application of the voltage may enable the electrochemical reduction of the metal oxide material to produce the mobile paths for diffusion of the oxygen ions.
With reference to
The one or more electrochemical cells 102 may be contained within a gas-tight enclosure 104, which may include an inlet 106 and an outlet 108. The inlet 106 is configured for providing, for example, a gas to the enclosure 104 for maintaining a gas pressure within the enclosure 104. Gases may be removed from the enclosure 104 via the outlet 108. In some embodiments, the gas comprises hydrogen, an inert gas, such as argon, helium, or a combination thereof. The enclosure 104 may include a furnace or other heating element for heating or maintaining a temperature of a molten salt electrolyte 110 in the electrochemical cell 102. Although
The electrochemical cell 102 may include a crucible 112 comprising a metal, glassy carbon, ceramic, a metal alloy, or another material. In some embodiments, the crucible 112 comprises a non-metallic material, such as alumina (Al2O3), magnesia (MgO), glassy carbon, graphite, boron nitride, another material, or combinations thereof. In other embodiments, the crucible 112 comprises a metal or metal alloy, such as, for example, nickel, molybdenum, tantalum, stainless steel, alloys of nickel and copper, alloys of nickel, chromium, iron, and molybdenum, alloys of nickel, iron, and molybdenum, and combinations thereof.
The molten salt electrolyte 110 may be disposed in the crucible 112. The electrochemical cell 102 may further include at least one counter electrode 114 (which may also be referred to as an anode) and at least one working electrode 116 (which may also be referred to as a cathode). The electrochemical cell 102 may also include a reference electrode 118 configured for monitoring a potential between the working electrode and a reference electrode in the electrochemical cell 102. A sheath 120 is disposed around at least a portion of one or more of the counter electrode 114, the working electrode 116, and the reference electrode 118. The sheath 120 may be configured to provide electrical insulation between the respective electrodes and the crucible 112. The sheath 120 comprises alumina (e.g., an alumina tube), magnesia (e.g., a magnesia tube), mullite (e.g., a mullite tube), or a combination thereof.
The counter electrode 114 may comprise, consist substantially of, or consist of any of the above-described counter electrode (anode) materials.
The reference electrode 118 may be in electrical communication with the counter electrode 114 and the working electrode 116 and may be configured to monitor the potential difference between the counter electrode 114 and the working electrode 116. Accordingly, the reference electrode 118 may be configured to monitor the cell potential of the electrochemical cell 102. The reference electrode 118 may be configured as a so-called “true reference electrode” or as a so-called “pseudo-reference electrode.” The reference electrode 118 may comprise, consist substantially of, or consist of any of the above-described reference electrode materials.
A power source, such as a potentiostat-galvanostat 122, or a direct current (DC) power supply, may be electrically coupled to each of the counter electrode 114, the working electrode 116, and the reference electrode 118. The potentiostat-galvanostat 122 may be configured to measure and/or provide an electric potential/current between the working electrode 116 and the reference electrode 118 while the current flows between the counter electrode 114 and the working electrode 116. The difference between the electric potential of the counter electrode 114 and the electric potential of the working electrode 116 may be referred to as the cell potential of the electrochemical cell 102.
The system 100 may be configured to reduce one or more non-stoichiometric metal oxides to a substantially pure metal (e.g., the metal in a substantially unoxidized state) or to a metal alloy. In some embodiments, the working electrode 116 is formed of and includes at least one oxide (e.g., at least one non-stoichiometric metal oxide) to be reduced in the electrochemical cell 102.
The working electrode 116 may be in electrical communication with a vessel (e.g., a basket 124) configured to carry one or more non-stoichiometric metal oxides to be reduced in the electrochemical cell 102. The basket 124 may comprise nickel, cobalt, iron, molybdenum, stainless steel, alloys of nickel and copper, alloys of nickel, chromium, iron, and molybdenum, alloys of nickel, iron, and molybdenum, another metallic material (e.g., titanium), a ceramic (e.g., boron nitride), or combinations thereof. In some embodiments, the basket 124 comprises nickel. In other embodiments, the electrochemical cell 102 does not include the basket 124 and the working electrode 116 comprises the non-stoichiometric metal oxide or a combination of non-stoichiometric metal oxides to be electrolytically reduced in the electrochemical cell 102. Stated another way, in some embodiments, the working electrode 116 comprises one or more non-stoichiometric metal oxides that are reduced to a metal (e.g., a substantially pure metal or a metal alloy) in the electrochemical cell 102. In some embodiments, the working electrode 116 consists essentially of the non-stoichiometric metal oxide, which may comprise one or more metals to be reduced.
At least one of the working electrodes 116 and the non-stoichiometric metal oxide in the basket 124 may comprise a non-stoichiometric metal oxide. The working electrode 116 and/or the one or more non-stoichiometric metal oxides in the basket 124 may comprise, consist substantially of, or consist of any of the above-described reference working electrode materials (e.g., non-stoichiometric metal oxide materials).
The working electrode 116 may consist substantially of or consist of a single non-stoichiometric metal oxide (e.g., any of the aforementioned non-stoichiometric metal oxides). Alternatively, the working electrode 116 may comprise, consist substantially of, or consist of a mixture of non-stoichiometric metal oxides, such as a mixture of a titanium oxide and a tantalum oxide; a mixture of a hafnium oxide and a titanium oxide; or a mixture of neodymium oxide (Nd2O3), boric oxide (B2O3), and iron oxide (Fc2O3).
In some embodiments, the metal oxide material comprises an unirradiated nuclear fuel, such as enriched/depleted uranium oxide (UO2, U3O8). In other embodiments, the metal oxide material comprises a spent nuclear fuel, such as spent uranium oxide (e.g., UO2). In some embodiments, the metal oxide comprises an oxide of more than one metal. Reduction of such oxides may form a metal alloy comprising the constituent metals of the metal oxides. In some embodiments, the non-stoichiometric metal oxide is disposed in the basket 124 and in electrical communication with the working electrode 116. In other embodiments, the working electrode 116 consists essentially of the non-stoichiometric metal oxide. In either such embodiments, the working electrode 116 comprises the non-stoichiometric metal oxide or the working electrode is connected to the non-stoichiometric metal oxide (e.g., the working electrode may be connected to pellets of the non-stoichiometric metal oxide disposed in the basket 124).
During the electrochemical reduction process, electrons are provided in the electrochemical cell 102 by provision of a current to the working electrode 116, such as through the potentiostat-galvanostat 122 or a DC power supply. The oxide ions generated at the working electrode 116 may be transported from the working electrode 116 to the counter electrode 114, via the electrolyte, under an applied electrical field (i.e., a polarization field between the counter electrode 114 and the working electrode 116), provided by the potentiostat-galvanostat 122. The oxygen ions enter the electrolyte, travel to the counter electrode, and discharge their electrons onto the counter electrode surface. The oxygen ions deposited on the surface of the counter electrode 114 subsequently get evolved as oxygen gas (O2) at the counter electrode 114. The electrons return to the working electrode 116 via the external circuit of the potentiostat-galvanostat to ionize the oxygen present in the non-stoichiometric metal oxide.
In use and operation, the non-stoichiometric metal oxide may be disposed in the electrochemical cell 102 (e.g., in the form of the working electrode) and in contact with the molten salt electrolyte 110. An electric potential/current may be applied between the counter electrode 114 and the working electrode 116, providing a polarization field and a driving force for moving oxide ions, from the working electrode 116, via the electrolyte, to the counter electrode 114, facilitating reduction of the non-stoichiometric metal oxide (e.g., the working electrode 116).
The resulting metal or metal alloy according to embodiments of the disclosure exhibits one or more of an increased porosity, an increased current efficiency, and an increased purity relative to a metal or a metal alloy formed by conventional electrochemical reduction processes, which use air/oxygen to sinter a metal oxide precursor and are limited to use of a stable metal oxide precursor (e.g., a stoichiometric metal oxide precursor). In embodiments of the disclosure, the resulting metal or metal alloy is a stable metal or stable metal alloy. The stable metal alloy prepared in accordance with the present disclosure may exhibit one or more of enhanced resistance to change or decomposition due to an internal reaction or due to the action of art, heat, light, or pressure, as compared to metals or metal alloys prepared with conventional processes. The purity of the resulting metal or metal alloy may depend on the oxide system(s). For example, for NiO/Cr2O3, the purity may range from about 99.5% to about 99.9%. For TiO2, the purity may range from about 99.0% to about 99.5%. For rare earth oxides, the purity may be greater than about 90%. The overall (cathodic) current efficiency may be increased by up to 60%. The process of forming the metal material according to embodiments of the disclosure may be used with a wider range of metal oxide materials since the process is not limited to using stable metal oxide materials (e.g., stable oxides, stoichiometric metal oxide materials). The resulting metal material may also be formed in a significantly decreased amount of time, which reduces loss of the metal material, into the electrolyte as a result of prolonged exposure to the electrolyte, compared to a metal or a metal alloy formed by conventional processes that use air/oxygen. The electrolyte may have a finite solubility of the reduced metal or metals. The metal material may be formed in about half the amount of time of a metal or a metal alloy formed by conventional processes that use air/oxygen. For instance, a conventional air sintering process reduces a stoichiometric metal oxide (e.g., a stable oxide such as titanium dioxide or tantalum pentoxide) to a metal (e.g., titanium or tantalum) exhibiting a residual oxygen content of about 3500 ppm in about 72 hours. In embodiments of the disclosure, a non-stoichiometric metal oxide (e.g., an anion-deficient metal oxide or defective metal oxide) is reduced to a metal. An anion deficient oxide may be considered an oxide having less oxygen as compared to a stoichiometric oxide. A non-stoichiometric metal oxide (e.g., anion-deficient metal oxide) reduced in accordance with embodiments of the disclosure may be reduced to a metal (e.g., titanium or tantalum) exhibiting a residual oxygen content of less than or equal to about 1200 ppm in about 20 hours. The reduced time also enables the metal material to be produced at a relatively higher throughput than those produced by conventional techniques. The resulting metal material may also exhibit a significantly less residual oxygen content compared to a metal or a metal alloy formed by conventional processes that use air/oxygen and a relatively longer time to undergo complete reduction to metal materials. The residual oxygen content of the metal material formed according to embodiments of the disclosure may be decreased by a margin of about 70% than that of the reduced metal produced from an air-sintered precursor. For example, a metal (e.g., titanium or tantalum, etc.) produced from an air-sintered precursor may exhibit a residual oxygen content of about 3500 ppm (about 0.35 weight percent). In contrast, a metal (e.g., titanium or tantalum) produced from a process in accordance with embodiments of the disclosure may exhibit a residual oxygen content of about 1200 ppm (about 0.12 weight percent), about 1000 ppm (about 0.10 weight percent), or about 800 ppm (about 0.08 weight percent). The process for forming the metal material from the metal oxide material according to embodiments of the disclosure also enables the metal material to be formed without making major changes to the manufacturing schedule. Therefore, the process is easily incorporated into existing manufacturing processes.
The metal material is recovered from the electrochemical cell 102 and is subsequently formed into an article (e.g., a metal article). The article may be formed from the metal material with minimal or no elaborate, post-reduction treatment acts (e.g., chemical treatments) conducted. Therefore, the article may be produced without conducting further treatments. The metal material may maintain the shape of the precursor non-stoichiometric metal oxide. The overall size (e.g., diameter) may be smaller than the starting non-stoichiometric metal oxide precursor material due to removal of oxygen during the reduction process. However, in embodiments of the disclosure, the metal material formed maintains the shape of the starting non-stoichiometric (e.g., anion-deficient) metal oxide precursor. That is, the metal material formed may be a near net shape material. Conditions can be maintained to prepare the metal material in the form of a powder. The powder metal material may be directly used for catalytic applications. The article including the metal material according to embodiments of the disclosure may be used in nuclear, aerospace, automotive, bio-medical, or defense industries, or in other industries in which a high purity, porous metal material is desired.
The following examples serve to explain embodiments of the present invention in more detail. These examples are not to be construed as being exhaustive or exclusive as to the scope of this invention.
Pellets of titanium oxide were prepared by mixing the titanium oxide with an organic binder and compressing (e.g., pelletizing) the mixture. The pellets were heated to a temperature of greater than about 1000° C. under a reducing gas that included 2.97% hydrogen gas in argon gas. The pellets were heated from 1 to 5 hours. The sintered pellets were examined under a scanning electron microscope (SEM) and their densities determined by conventional techniques. For comparison, control pellets were heated to a temperature of greater than about 1000° C. in an air environment.
The sintered pellets were threaded into a nickel wire and immersed into a CaX2—CaO molten salt electrolyte, where X is Cl2 or Br2, to electrochemically reduce the titanium oxide. The electrochemical reduction was conducted at a temperature between about 800° C. and about 950° C. A three electrode setup including the sintered pellets as the working electrode, a platinum group metal rod as the anode, and a glassy carbon rod as the reference electrode was used to conduct the electrochemical reduction experiment.
The pellets exposed to the reducing gas shrunk in diameter and thickness due to the removal of the oxygen.
About 95% of the sintered pellets exposed to the reducing gas were reduced in comparatively less time than that used to achieve a comparable reduction in the control pellets. The sintered pellets exposed to the reducing gas were reduced in half the amount of time used to achieve a comparable reduction in the control pellets.
Pellets of tantalum pentoxide were prepared by mixing the tantalum pentoxide with an organic binder and compressing (e.g., pelletizing) the mixture. The pellets were heated to a temperature of greater than about 1000° C. under a reducing gas that included 2.97% hydrogen gas in argon gas. The pellets were heated for a period of from 1 to 5 hours. SEM micrographs showed an increased degree of porosity in the sintered pellets that were exposed to the reducing gas relative to control pellets that were sintered in an air environment.
The sintered pellets were threaded into a nickel wire and immersed into a CaCl2—CaO molten salt electrolyte to electrochemically reduce the tantalum oxide. The electrochemical reduction was conducted at a temperature between about 850° C. and about 950° C. A three electrode setup including the sintered pellets as the working electrode, a platinum group metal rod as the anode, and a glassy carbon rod as the reference electrode was used to conduct the electrochemical reduction. The oxygen content of the pellets after reduction was less than 1500 ppm and the current efficiency was between 50% and 70%.
The reduced pellet, after removal from the electrochemical cell, was observed to have been reduced both in diameter and thickness. The reduced pellet was kept immersed in distilled water to wash off the solidified salt. The salt was dissolved and, in that process, the solid pellet turned into a fine, greyish black powder. This is the only metal oxide which turned powdery. Without being bound by any theory, the formation of the powder may be due to the very high melting point of tantalum, which is 2996° C.
The embodiments of the disclosure described above and illustrated in the accompanying drawings do not limit the scope of the disclosure, which is encompassed by the scope of the appended claims and their legal equivalents. Any equivalent embodiments are within the scope of this disclosure. Indeed, various modifications of the disclosure, in addition to those shown and described herein, such as alternate useful combinations of the elements described, will become apparent to those skilled in the art from the description. Such modifications and embodiments also fall within the scope of the appended claims and equivalents.
This application claims the benefit under 35 U.S.C. § 119 (e) of U.S. Provisional Patent Application Ser. No. 63/494,653, filed Apr. 6, 2023, the disclosure of which is hereby incorporated herein in its entirety by this reference.
This invention was made with government support under Contract Number DE-AC07-05-ID14517 awarded by the United States Department of Energy. The government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 63494653 | Apr 2023 | US |
