The invention relates to non-stoichiometric substances and more particularly to nanostructured non-stoichiometric substances and products incorporating such substances.
Most compounds are prepared as stoichiometric compositions, and numerous methods of preparing substances for commercial use are motivated in objective to create stoichiometric compounds. For example, producers of titania fillers, copper oxide catalysts, titanate dielectrics, ferrite magnetics, carbide tooling products, tin oxide sensors, zinc sulfide phosphors, and gallium nitride electronics all seek stoichiometric compositions (TiO2, CuO, BaTiO3, NiFe2O4, TiC, SnO2, ZnS, and GaN, respectively).
Those skilled in the art will note that conventional powders of oxides and other compounds, when exposed to reducing atmospheres (e.g. hydrogen, forming gas, ammonia, and others) over a period of time, are transformed to non-stoichiometric materials. However, the time and cost of doing this is very high because the inherent diffusion coefficients and gas-solid transport phenomena are slow. This has made it difficult and uneconomical to prepare and commercially apply stable non-stoichiometric forms of materials to useful applications.
Limited benefits of non-stoichiometric materials have been taught by others; for example, Sukovich and Hutcheson in U.S. Pat. No. 5,798,198 teach a non-stoichiometric ferrite carrier. Similarly, Menu in U.S. Pat. No. 5,750,188 teaches a method of forming a thin film of non-stoichiometric luminescent zinc oxide. The film is a result of a thermodynamically favored defect structure involving non-stoichiometric compositions where the non-stoichiometric deviation is in parts per million.
This invention includes several methods of making non-stoichiometric submicron and nanostructured materials and devices from both stoichiometric and non-stoichiometric precursors. This invention also includes methods of making stoichiometric materials and devices from non-stoichiometric precursors. In one aspect, the invention includes an improved sintering technique utilizing submicron non-stoichiometric powders. The invention also includes a variety of other applications for submicron non-stoichiometric materials, including catalysis, photonic devices, electrical devices and components, magnetic materials and devices, sensors, biomedical devices, electrochemical products, and energy and ion conductors.
In one aspect, this invention includes a variety of methods of producing a non-stoichiometric material. According to one method, a submicron powder of a stoichiometric material is transformed into a non-stoichiometric powder. The submicron powder may also be a nanopowder. If desired, the submicron non-stoichiometric powder may be sintered into a bulk substance. This invention excludes from its scope the non-stoichiometry that naturally results from the randomly occurring thermodynamic defects in a bulk crystal of the theoretical stoichiometry which are typically on the order of a few hundred parts per million. As used herein, non-equilibrium means thermodynamic non-equilibrium.
According to another method, a non-stoichiometric submicron material is produced by quenching a high-temperature vapor of a precursor material to produce a non-stoichiometric submicron powder. A vapor stream of the high temperature vapor flows from an inlet zone, and this stream is passed through a convergent means to channel the vapor stream through an area where flow is restricted by controlling the cross-section of the flowing stream. The vapor stream is channeled out of the flow restriction through a divergent means to an outlet pressure which is smaller than the inlet pressure. This quenches the vapor stream. The inlet and outlet pressures are maintained, creating a pressure differential between them. The pressure differential and the cross-section of the flow restriction are adapted to produce a supersonic flow of the vapor stream. This method may further comprise sintering the resulting powder.
According to yet another method, a nanoscale starting material comprising more than one element is provided. At least one of these elements is an electropositive element. A dopant element with valency different than the electropositive element is added, and the mixture is heated to a selected temperature, preferably greater than the solid state reaction temperature, for a time sufficient to allow intermingling of the dopant element and the given electropositive element.
According to still another method, two nanopowders are mixed in a ratio selected to produce a desired non-stoichiometric composition. The first nanopowder comprises a plurality of materials, and the second comprises a subset of those materials. The materials comprising the first nanopowder maybe metallic, semimetallic, non-metallic, or any combination thereof. The mixture is heated in a selected atmosphere to a temperature to produce a solid state reaction. The atmosphere may participate in the solid state reaction. This invention also includes the materials produced via the above methods.
In another aspect, this invention includes a submicron non-stoichiometric material where the value for a selected physical property of the submicron non-stoichiometric material is greater than 10% different from that for a stoichiometric form of the submicron non-stoichiometric material. Alternately, the relative ratios of the components of the material differ by more than 1% from the stoichiometric values, preferably 2% from the stoichiometric values, and more preferably 5%. The material may be a nanomaterial or a nanopowder.
This invention also includes a submicron material wherein a domain size of the material is less than 500 nm, and the material is non-stoichiometric. Preferably, the domain size is less than 100 nm. Alternately, a domain size may be less than 5 times the mean free path of electrons in the given material, or the mean domain size maybe less than or equal to a domain size below which the substance exhibits 10% or more change in at least one property when the domain size is changed by a factor of 2. The material may be a powder or a nanopowder.
In another aspect, this invention includes a method of determining the non-stoichiometry of a material. A stoichiometric form of the material and the material whose stoichiometry is to be ascertained (the xe2x80x9cunknownxe2x80x9d material) are heated separately in a reactive atmosphere to 0.5 times the melting point of the material. The weight change per unit sample weight for the unknown material is monitored. In addition, the weight change per unit sample weight of the unknown material is compared to the weight change per unit sample weight of the known material.
In another aspect, this invention includes a method of conducting combinatorial discovery of materials where non-stoichiometric forms of materials are used as precursors.
In another aspect, this invention includes a method of making a non-stoichiometric nanoscale device by fashioning a non-stoichiometric nanoscale material into a device. Alternately, a device is fashioned from a stoichiometric material and the stoichiometric material converted into a non-stoichiometric form. The stoichiometric material may be an electrochemical material, a photonic material, or a magnetic material. The non-stoichiometric material may be electroded; and the electrode may comprise a non-stoichiometric material. This invention also includes stoichiometric devices with non-stoichiometric electrodes. The non-stoichiometric materials may further be a nanomaterials.
In another aspect, this invention includes a method of producing a stoichiometric material from a non-stoichiometric powder. The powder is processed into the shape desired for a stoichiometric material and further processed to produce stoichiometric ratios among its components. This invention also includes a method of producing a stoichiometric device via the same method.
In another aspect, this invention also includes an improved method of producing sintered materials. A submicron stoichiometric powder is formed into a green body. The green body is sintered at a selected densification rate and a selected temperature which are lower than those required to sinter larger, stoichiometric powders. This method may further comprise converting the sintered material to a stoichiometric form or stabilizing the non-stoichiometric sintered material by the addition of a protective coating, secondary phase, or stabilizer. In this method, the submicron non-stoichiometric powder may also be nanopowders.
This invention also includes a method of producing an improved catalyst. A nanopowder comprising indium tin oxide and alumina are pressed into pellets. The pellets are reduced in a reducing atmosphere to form a catalyst which can promote the formation of hydrogen from 12% methanol vapor at 250xc2x0 C. This invention also includes the improved catalyst prepared by this method.
In another aspect, this invention includes a method of producing an improved photonic material. A high-temperature vapor of a precursor material is quenched from a gas stream comprising hydrogen and argon to produce a non-stoichiometric submicron powder such that the absorption of selected wavelengths is more than doubled with respect to that of a stoichiometric from of the precursor. In this method, the precursor material may be stoichiometric ITO; the selected wavelengths would be greater than 500 nm. In addition, this invention includes an improved non-stoichiometric photonic material produced by this process and exhibiting enhanced absorption of selected wavelengths of electromagnetic radiation in comparison to a stoichiometric form of the material.
In another aspect, this invention includes a method of producing an improved electric device. Titanium oxide nanopowders are heated in an ammonia atmosphere to produce a non-stoichiometric oxynitride of titanium. The resulting device may also be part of an electrical conductor. This invention also includes the improved electrical device produced by this method.
This invention also includes a variety of methods of making improved magnetic materials and devices. According to one method, a nickel zinc ferrite material is sintered to near theoretical density and heated in a reducing atmosphere at an elevated temperature such that the resulting material exhibits higher magnetic loss than the stoichiometric starting material. The atmosphere may comprise 5% H-95% Ar and the temperature may be 800xc2x0 C.
According to another method, a mixture of two stoichiometric nanopowders is produced from manganese ferrite and nickel zinc ferrite powders. These two powders are pressed together, sintered, and wound. The method may further comprise pressing the two nanopowders with a binder, preferably 5% Duramax. This invention also includes the magnetic devices and materials produced by these methods.
In another aspect, this invention includes methods of making a non-stoichiometric resistor. In one method, the resistor is produced from a stoichiometric submicron material and transformed to a non-stoichiometric form. In another method, the resistor is produced from non-stoichiometric SiCx nanopowders. The nanopowders are sonicated in polyvinyl alcohol and screened printed on a alumina substrate. The resulting resistor element is to produce a resistor having a resistance less than 1 megaohm. Platinum or silver dopants may be added to the sonicated mixture. This invention also includes the improved resistors produced via these methods and arrays of resistors produced via these methods.
In another aspect, this invention also includes a method of producing an improved sensor device. A non-stoichiometric nanopowder is sonicated in a solvent to form a slurry. The slurry is brushed onto screen-printed electrodes and allowed to dry at to remove the solvent. A dissolved polymer may also be included in the slurry. The screen-printed electrodes may be gold electrodes on an alumina substrate. The screen may be made from stainless steel mesh at least 8xc3x9710 inches in size, with a mesh count of 400, a wire diameter of 0.0007 inches, a bias of 45xc2x0, and a polymeric emulsion of 0.0002 inches.
In another aspect, this invention includes an improved sensor device prepared from a screen printable paste. A nanopowder and polymer are mechanically mixed; a screen-printing vehicle is added to the mixture and further mechanically mixed. The mixture is milled and screen-printed onto prepared electrodes. The paste is allowed to level and dry. This invention also includes the improved sensor devices produced by the above processes.
This invention, in a further aspect, includes a method of making an improved biomedical orthopedic device. A feed powder comprising a non-stoichiometric Tixe2x80x94Taxe2x80x94Nbxe2x80x94Zr alloy is milled under non-oxidizing conditions. The milled powder is mixed with a binder dissolved in a solvent and allowed to dry. The mixture is then pressed and incorporated into a biomedical device. This invention also includes a biomedical material comprising a non-stoichiometric submicron powder. In addition, this invention includes a biomedical material produced by this process wherein the powder is a nanopowder.
This invention, in another aspect, includes a method of preparing an improved electronic component. A non-stoichiometric nanoscale material is mixed with a screen printing material and the resulting paste screen-painted on an alumina substrate. The paste is wrapped up and dried on a heated plate and further screen-printed with silver-palladium to form a conducting electrode. The silver-palladium is dried rapidly on a heated plate and the two films co-fired.
In another aspect, this invention includes an improved electrochemical material comprising a submicron non-stoichiometric material. The material has excess Gibbs free energy in comparison to larger grained materials. In addition, the material exhibits increased solute diffusion, lower phase transformation temperatures, and high compressive toughness.
In another aspect, this invention includes a method of making an improved energy and ion conducting device. A stoichiometric nanoscale starting powder is reduced at a temperature between 500xc2x0 C. and 1200xc2x0 C. in a forming gas to yield non-stoichiometric nanopowders. The powders are pressed into discs, sintered, and coated with a cermet paste comprising equal parts silver and a stoichiometric nanoscale form of the starting powder. Platinum leads are then attached to the cermet paste. Preferably, the cermet paste comprises silver and a non-stoichiometric version of the starting powder. The starting powder may be yttria-stabilized cubic zirconia, other metal oxides, a perovskite material, or another group IV oxide. This invention also includes the improved energy and ion conducting device produced by this method. In addition, it includes an ion and energy conducting device wherein the ion conductor is produced from nanostructured beta alumina, NASICON, lithium nitride, LISICON, silver iodide, Rb4Cu16I7Cl13, a polymer, or a perovskite.
In another aspect, this invention includes an improved dopant for semiconductor materials where the dopant comprises a non-stoichiometric nanocrystalline powder. The grain size of the non-stoichiometric nanocrystalline powder maybe less than 80 nm, preferably 40 nm, and more preferably 10 nm. The non-stoichiometric nanocrystalline powder may include one or more materials selected from the group comprising Ta2/3O0.9, Nb2/5O0.74, NiO0.98, Mn1/2O0.9, Bi2/3O0.45, Cu1.9O, TiO1.1, SiO1.55, and V2/5O0.975.
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