Tuesday, May 4, 2010

Tungsten Nanoparticles May Be As Versatile As Carbon Nanotubes

Nanoparticles of  tungsten, tungsten nanoalloys and nanocompounds, methods of manufacturing tungsten nanoparticles, and applications of tungsten nanoparticles in electronics, in optical devices, in photonics, as reagents for fine chemical synthesis, as pigments, and as catalysts earned U.S. Patent 7,708,974 for PPG Industries Ohio, Inc. (Cleveland, OH).

Tapesh Yadav  (Longmont, CO) developed methods of producing very fine, high purity powders of  tungsten (W) in high volume, low-cost, and reproducible quality.

Pigments

Nanoparticles comprising tungsten containing multi-metal oxides offer some surprising and unusual benefits as pigments. Nanoparticles are smaller than the visible wavelengths of light which leads to visible wavelengths interacting in unusual ways with nanoparticles compared to particles with grain sizes much bigger than the visible wavelengths (400-700 nm). The small size of nanoparticles can also lead to more uniform dispersion.

One of the outstanding process challenges for manufacturing inorganic pigments is the ability to ensure homogeneous lattice level mixing of elements in a complex multi-metal formulation. One of the features of the patented process is its ability to prepare complex compositions with the necessary homogeneity. Tungsten nanocompounds ideally are suited for creating color and making superior performing pigments with nanoparticles comprising tungsten.

Additives

Nanoscale tungsten comprising substances are useful lubricating additives. A non-limiting illustration is tungsten disulfide nanoparticles. The small size of tungsten disulfide nanoparticles enables thinner films in certain embodiments offering reduced costs at higher performance. Such lubricating nanoparticles offer ability to distribute forces more uniformly.

In certain embodiments such as high precision, tight gap moving surfaces, lubricating additives may be added to the lubricating fluid or oils to improve the life or motor or engine. The unusual characteristic that makes lubricating nanoparticle additives useful is that the particle size enables by nanotechnology can be less than the naturally occurring characteristic roughness sizes. The nanoparticles can enter and buffer (or reside) in crevices, troughs thereby reducing the damaging internal pressures, forces and inefficient thermal effects.

These additives can be dispersed in existing or novel lubricating formulations and thereby provide an easy way to incorporate the benefits of nanotechnology. Tungsten disulfide, molybdenum disulfide, molybdenum tungsten sulfide and such inorganic or organic nanoparticle composition are useful lubricating additives elsewhere as well, e.g. shaving blades and any surface that requires minimization of the adverse effects of friction.

Biochemical Analytical Agent

Sodium tungsten oxide nanoparticles, in high purity form are useful in biochemical analysis. The high surface area of nanoparticles comprising tungsten, particularly when the mean particle size is less than 100 nanometers, make them useful in these applications.

Sulfur and Carbon Accelerating Analytical Agent

Tungsten nanoparticles, in metallic form are useful in the analysis of carbon and sulfur by combustion in an induction furnace. The high surface area and surface activity of nanoparticles comprising tungsten, particularly when the mean particle size is less than 100 nanometers, make them useful in these applications.

Electrical & Lighting Applications

Tungsten nanomaterials comprising offer several unusual benefits as electron emitters. These benefits are a consequence of (a) the small size of nanoparticles which can enable very thin film devices, (b) high surface area, which can lower the sintering temperatures and sintering times, (c) inherently low vapor pressure of tungsten metal even at high temperatures and (d) unusual quantum confinement and grain boundary effects.

These properties can be used to prepare improved electron emitting devices and electrical contacts. Photocopiers, facsimile machines, laser printers and air cleaners can benefit from charger wires prepared from tungsten comprising nanomaterials. Other nanodevices that can be prepared from nanoscale powders comprising tungsten include electrodes, chemical sensors, biomedical sensors, phosphors, and anti-static coatings. Tungsten comprising nanomaterials also offer novel compositions for chemical mechanical polishing applications.

Nanomaterials comprising tungsten are particularly useful at direct heated cathode or heater coils for indirectly heated cathodes in cathode ray tubes, displays, x-ray tubes, klystrons, magnetrons for microwave ovens and electron tubes. Multimetal nanomaterial compositions comprising tungsten include those based on rare earths and thoria for high intensity discharge lamps and welding electrodes. X-ray device anodes can also benefit from the low vapor pressure and thermal conductivity of tungsten comprising nanomaterials.

The unusual combination of vapor pressure, electrical conductivity and electronic properties make nanomaterial compositions comprising tungsten useful as substrate for high power semiconductor rectifying devices, high voltage breakers (e.g. W--Cu, W--Ag contacts). Various forms of infiltrated tungsten comprising nanocomposites are useful for these applications. High temperature furnace parts such as heating coils, reflectors, thermocouples can also benefit from the quantum confined and low vapor pressure characteristics of nanomaterials comprising tungsten.

Tungsten nanomaterials are useful for lighting applications (incandescent lamps). Illustrative applications include household lamps, automotive lamps, and reflector lamps for floodlight or projector applications. Specialty lamps can also benefit from the nanotechnology taught herein in applications such as, but not limited to, audio-visual projectors, fiber-optical systems, video camera lights, airport runway markers, photoprinters, medical and scientific instruments, and stage or studio systems.

Electronics Applications

Tungsten nanomaterials are useful as phosphors and electronic materials. These benefits are a consequence of (a) the small size of nanoparticles which can enable very thin film devices, (b) high surface area which can lower the sintering temperatures and sintering times, (c) inherently low vapor pressure of tungsten metal even at high temperatures, (d) significant thermal and electrical conductivity, and (e) unusual quantum confinement and grain boundary effects. These properties can be used to prepare improved phosphors for x-rays (e.g. calcium tungsten oxide, magnesium tungsten oxide). Improved and more cost effective heat removal components comprising of tungsten and copper based heat sinks can be prepared from nanomaterials. Tungsten comprising nanomaterials inks (water, solvent or UV curable), adhesives and pastes can be useful in developing electrodes and conductors for ceramic circuit board and other applications.

For silicon based semiconductor devices, tungsten nanomaterials offer a close thermal coefficient of expansion. In combination with other useful properties mentioned above, tungsten and complex compositions comprising tungsten offer materials that can help achieve a thermal coefficient of expansion similar to compositions comprising silicon and metals used in microelectronics. These properties can be used to prepare improved microelectronic components. Tungsten comprising nanomaterials inks and pastes can be useful in preparing improved DRAM chips, other silicon devices, and liquid crystal display products.

Catalysts

Tungsten nanomaterials such as oxides, sulfides and heteropoly complexes are useful catalysts for a number of chemical reactions. For example, they can be used in hydration, dehydration, hydroxylation, and epoxidation reactions as catalysts or promoters. In one embodiment, a method for producing catalysts or promoters comprises (a) preparing nanoscale powders comprising tungsten such that the surface area of the said powder is greater than 25 square meters per gram, in some embodiments greater than 75 square meters per gram, and in some embodiments greater than 150 square meters per gram; and (b) reducing the powder in a reducing environment (or activating the powder in any other way) and then conducting a chemical reaction in the presence of the nanoscale powders comprising doped or undoped tungsten compound. In some embodiments, a further step of dispersing the nanoscale powders in a solvent and then depositing these powders onto a substrate from the dispersion may be employed before chemical reaction is conducted.

The catalyst powders can be combined with zeolites and other well-defined porous materials to enhance the selectivity and yields of useful chemical reactions.

Optics and Phosphors

Non-stoichiometric tungsten nanoparticles offer several unusual benefits as phosphors and for detector applications. These benefits are a consequence of one or more of the following characteristics (a) small size, (b) high surface area, (c) dispersability in various media, inks, and solid matrices, (e) unusual and complex combinations of density, vapor pressures, work functions, and band gaps. The advantages of phosphors and detectors comprising tungsten-containing nanoparticles are (a) high dots per inch density, (b) ability to form homogeneous products, and (c) the ability to prepare very thin films thereby reducing the raw material required for same or superior performance. Nanoparticles can also be post-processed (calcination, sintering) to grow the grain to the optimal size in order to provide the brightness level, decay time and other characteristics desired.

Multi-metal compositions (two, three, four, or more metals) comprising tungsten are used in certain embodiments. A specific illustration of fluorescent composition is calcium tungstate. These phosphor nanopowders can be used for scintillation counters, display applications, lamps, fluorescent bulbs, light emitting devices, markers, security pigments, fabric pigments, luminous paints, toys, special effects, etc.

Tungsten comprising nanoparticles are useful in forming thin films comprising tungsten that lose oxygen in bright light thereby becoming bluish and filtering light; these films reoxidize in darkness thereby becoming clear. One of the many useful properties of nanomaterials comprising tungsten is the ability of tungsten to lose oxygen easily (e.g. WO3--W20O58 transition).

Interstitial Compounds

Interstitial compounds comprising tungsten (e.g. carbide, nitrides, borides, silicides) offer several unusual benefits in hard, refractory applications. These benefits are a consequence of one or more of the following characteristics (a) size, (b) hardness, (c) size confinement, (e) unusual and complex combinations of density, vapor pressures, and physical properties. Nanoparticles can also be post-processed (calcination, sintering) to grow the grain to the optimal size in order to provide other characteristics as desired. Interstitial nanomaterial compositions comprising tungsten (and other metal(s)) are useful in cutting tools, structural elements of kilns, turbines, engines, sandblast nozzles, protective coatings and the like.

Reagent and Raw Material for Synthesis

Nanoparticles comprising tungsten such as tungsten oxide and tungsten containing multi-metal oxide nanoparticles are useful reagents and precursors to prepare other compositions of nanoparticles comprising tungsten. In a generic sense, nanoparticles comprising tungsten are reacted with another substance, i.e., reagent, such as an acid, alkali, organic, monomer, ammonia, reducing fluids, oxidizing fluids, halogens, phosphorus compounds, chalcogenides, biological materials, gas, vapor or solvent; the high surface area of nanoparticles facilitates the reaction and the product resulting from this reaction is also nanoparticles. The reagent can take any suitable form and can comprise nitrogen, a halogen, hydrogen, carbon, or oxygen.

These product nanoparticles can then be suitably applied or utilized to catalyze or as reagents to prepare other fine chemicals for a wide range of applications. for example tungstate nanoparticles are useful materials for optical, electronic, catalyst, pigment, and other applications. Ceramics, corrosion resistance, and fire inhibition formulations can also benefit from the unusual surface activity, small size, and other properties of tungstate nanomaterials



The figure illustrates the PPG process for forming tungsten nanoparticles. 

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