NANO-COMPOSITE STRUCTURE AND PROCESSES MAKING OF

Abstract

A nano-composite structure comprises of an amorphous matrix with embedded nano-crystallites. The nano-crystallites are precipitated from the amorphous matrix via heat treatment of a solution mixture of metal salts or metalorganic compounds to an appropriate temperature range and with a suitable duration, or heating of a mixture of non-crystalline compounds. The nano-crystallites are self-assembled in the amorphous matrix without forming agglomerates or distinguished grain boundaries. The nano-composite structure can be used for transparent display, transparent optical ceramics, protection armor, nuclear protection, pulsed power, high voltage electronics, high energy storage system and high power microwave systems.

Claims

1. A nano-composite structure comprising an amorphous matrix with embedded nano-crystallites and the said matrix and the said nano-crystallites are made of the same chemical elements.

2. The nano-composite structure in claim 1 exhibits no distinguishable crystalline grain boundaries between the said matrix and the said nano-crystallites.

3. The nano-composite structure in claim 1 exhibits no or small refractive index difference between the said matrix and the said nano-crystallites.

4. The nano-composite structure in claim 1 is dense and has few voids.

5. The crystalline structure of the nano-crystallites in claim 1 can be perovskite, pyrochlore, or combination of two, but not limited to.

6. The size of the nano-crystallites in claim 5 is between 1 nm and 300 nm.

7. The materials of the said matrix and the said nano-crystallites in claim 1 can be PbTiO.sub.3, PLZT, PLT, PT, PMNPT, BaTiO.sub.3, BST, PZNPT, SrTiO.sub.3, or their combinations, but not limited to.

8. The materials in claim 7 further consists of rare earth elements of Er, Yb, La, Ho, Tm, Nd, or their combinations, but not limited to.

9. The nano-composite structure in claim 1 is transparent.

10. The nano-composite structure in claim 1 exhibits photo luminescence.

11. The nano-composite structure in claim 9 is for display application.

12. The nano-composite structure in claim 7 is for applications such as, but not limited to, high voltage device, pulsed power, high energy storage, high power microwave systems and power electronics.

13. The nano-composite structure in claim 1 is in a film that formed on a substrate with thickness between 100 nm to 50 m.

14. The substrate in claim 13 consists of, but not limited to, a glass, sapphire, quartz, silicon, polymer, plastic, or polycrystalline alumina.

15. The nano-composite structure in claim 1 is fabricated by a solution coating method with the same chemical elements consists of: Mixing individual metal-organic compound at the desired cation ratios to form a coating solution; Coating the solution on a substrate to form a wet film; Heating the wet film to decompose the metal-organic compounds to produce an amorphous inorganic film on the substrate; Further heating the amorphous matrix to let the nano-crystallites precipitated from the matrix, and Repeating the process to form desired film thickness.

16. The metal-organic compounds in claim 15 are from metal salts and/or alkoxides.

17. The coating process in claim 15 is dip-coating, spray coating or spin-coating.

18. The heating temperature in claim 15 is from 300 C. to 650 C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] FIG. 1 is a schematic diagram illustrating of a nano-composite structure in accordance with the present invention;

[0022] FIG. 2 is a schematic illustration of a solution coating apparatus used for fabricating the nano-composite structure in this invention;

[0023] FIG. 3 is a flow chart explaining a process of manufacturing the films;

[0024] FIG. 4 is showing, a typical XRD spectrum indicating the formation process of the nano-composite structure;

[0025] FIG. 5 is showing a typical TEM image of die nano-composite;

[0026] FIG. 6 is showing upconversion emission spectra of the PLZT: 5% Er, Yb glass ceramic thin films with different Yb doping concentrations;

[0027] FIG. 7 is showing a transmission spectrum of a nano-composite upconversion film;

[0028] FIG. 8 is showing an AFM image of a typical nano-composite thin film surface.

[0029] FIG. 9 is showing a cross-section image of a typical nano-composite thin film coated on c-sapphire substrate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] Shown in FIG. 1 is a preferred embodiment according to the present invention. The nano-composite structure is comprising of an amorphous matrix with embedded nano-crystallites, which are precipitated and self-assembled in the matrix.

[0031] In a preferred embodiment, the nano-composite films are fabricated by a solution coating method. The coating apparatus used for coatings in this invention is shown schematically in FIG. 2. it consists of three major parts: a driving mechanism 21, a vertical tube furnace 22, and a computerized controller 23. This apparatus is capable of multiple cycles that consist of immersing, withdrawing, drying, annealing and cooling stages. The number of cycles, the number of stages in a cycle and the motion and/or duration of each stage can all be programmed into the computer. The system is also equipped with an automated exchanger 24 for solutions of different precursors. A unique advantage of the coating process is that it can be used to grow multilayer (or superlattice) materials conveniently by dipping the substrate into different chemical precursors each time.

[0032] In this technique, solutions of individual metal-organic compounds are mixed at the desired cation ratios to form a coating solution. This coating solution is deposited on a substrate by dip-coating, or spray coating, or spin coating, to produce a wet film, which is then heated to first remove any solvent that did not evaporate during the deposition step and then to decompose the metal-organic compounds to produce an inorganic film. For most applications, the first consideration for an adequate film forming process is the ability to produce a final crack-free film. Solution-derived films tend to form cracks with increasing thickness owing to the high volume shrinkage as organic materials are removed during the firing process. This is especially true for films requiring a high-temperature treatment for crystallization. Not only is there another volume shrinkage accompanying the amorphous-to-crystalline transformation but factors like thermal expansion mismatch between the film and the substrate also become more severe as the processing temperature increases.

[0033] Similar solution coating methods include spin-coating and spray coating.

[0034] A typical flow chart for depositing films front solutions by multiple dipping is shown in FIG. 3. The temperature cycling nature of the solution coating process (by repeated heating and cooling for each layer) significantly reduces the stress due to thermal expansion mismatch between the substrate and film materials.

[0035] In one of the preferred embodiments, the nano-composite material PLZT has a general formula Pb.sub.1zLa.sub.z[Er.sub.yYb.sub.w(Zr.sub.xTi.sub.1x).sub.13(y+w)/4z/4]O.sub.3, wherein x is between about 0.05 and about 0.95, y is between about 0 and about 0.10, z is between about 0 and about 0.15, and w is between 0 and 0.50. In one especially preferred materials of the present invention, x is between about 0.55 and about 0.85, y is between about 0.01 and about 0.05, z is between about 0.07 and 0.12, and w is between 0.1 and 0.5.

[0036] PLZT is the most common electro-optic ceramic material. PLZT materials can be formed to be substantially transparent to light having wavelengths in the range of visible and near infrared. Properties of PLZT compositions can be tuned by adjusting the relative amounts of lead, lanthanum, zirconium and titanium.

[0037] The coating solution for PLZT thin film is made by dissolving lanthanum acetate, lead acetate and zirconium acetate in methanol and D.I. water, with a small amount of acetic acid as stabilizer. Then stoichiometric amount of titanium diisopropoxide bis(acetylacetonate) (TLAA) is added into the solution. The viscosity and wettability are adjusted with methanol and 2-methoxyethanol. The concentration of the precursor solution is between 0.01 to 0.5M (mol/L).

[0038] With above solution, the thin film is coated on a glass substrate, the dipping speed is between 0.1-20 mm/second, preferring between 0.5 and 10 mm/second. The firing speed is between 0.1-20 mm/second, with a preferred speed between 0.5 and 10 mm/second. The nano-composite structure with a PLZT based composition is formed between temperatures about 400 C. and 600 C. The thickness of the coatings is between 100 nm to 50 m.

[0039] In another preferred embodiment, the nano-composite films are lead titanate (PT) based nano-composite. The precursor solution for preparing the lead titanate (PT) nano-composite films is obtained by dissolving stoichiometric amount of lead acetates and titanium diisopropoxide bis(acetylacetonate) (TIAA) in methanol and acetic acid. 2-methoxyethanol and methanol are used to adjust the viscosity. Concentrations of the solutions are between 0.01 and 0.5 M (mol/L).

[0040] With above solution, the thin film is coated on a glass substrate, the dipping speed is between 0.1-20 mm/second, preferring between 0.5 and 10 mm/second. The firing speed is between 0.1-20 mm/second, with a preferred speed between 0.5 and 10 mm/second. The nano-composite structure with a PT based composition is formed between temperatures about 300 C. and 500 C. significant lower than that for the PLZT materials. The thickness of the coatings is between 100 nm to 50 m.

[0041] In yet another preferred embodiment, the composition of the nanocomposite material is rare earth elements doped or codoped PLZT (lead lanthanum zirconate titanate). In a specially preferred embodiment, it is Er and Yb ions co-doped PUT.

[0042] The coating solution for Er and Yb ions co-doped PUT thin film is made by dissolving lanthanum acetate, lead acetate and zirconium acetate in methanol and D.I. water, with a small amount of acetic acid as stablizer. Then stoichiometric amount of titanium diisopropoxide bis(acetylacetonate) (TIAA), erbium/ytterbium nitrate are added into the solution. The viscosity and wettability are adjusted with methanol and 2-methoxyethanol. The concentration of the precursor solution is between 0.01 to 0.5M (mol/L).

[0043] With above solution, the thin film is coated on a glass substrate, the dipping speed is between 0.1-20 mm/second, preferring between 0.5 and 10 mm/second. The firing speed is between 0.1-20 mm/second, with a preferred speed between 0.5 and 10 mm/second. The Er/Yb doped PLZT nano-composite structure is formed between temperatures about 400 C. and 600 C. The thickness of the coatings is between 100 nm to 50 m.

[0044] Lead lanthanum titanate (PLT), or lead titanate (PT) can be used instead of the PLZT. The doping element also can be Tm and Yb ions, or Ho, Yb and Nd ions, but not limited to

[0045] Sapphire, quartz, silicon, polymer, plastic, polycrystalline alumina, but not limited to, may also be used as substrate.

[0046] The composition of the nano-composite materials can also consist one of or combination of the following materials, but not limit to: lead magnesium niobate-lead titanate (PMN-PT, (1x)PbMn.sub.1/3Nb.sub.2/3O.sub.3xPbTiO.sub.3), lead zinc niobate-lead titanate (PZN-PT, (1x)PbZn.sub.1/3Nb.sub.2/3O.sub.3-xPbTiO.sub.3), barium titanate (BaTiO.sub.3), barium strontium titanate (BST, Ba.sub.xSr.sub.1xTiO.sub.3), strontium titanate (SrTiO.sub.3), or lanthanum or other elements substituted these compounds.

[0047] Shown in FIG. 4 are XRD patterns of a PLZT:5% Er, 10% Yb thin films annealed at different temperatures, illustrating the formation process of the nano-composite structure. Films annealed at 350 C. possess an amorphous phase and have no visible upconversion luminescence detected under the 970 nm laser excitation. A nano-sized pyrochlore phase is identified in the samples annealed at temperatures from 450 C. to 550 C. These samples show large upconversion effect under a 970 nm laser excitation and exhibit high transparency. A mixture of perovskite phase and pyrochlore phase is formed in the films annealed at 600 C. The co-existence of the two phases makes the films opaque. When the annealing temperature is raised up to 650 C., the pyrochlore phase disappears and a pure perovskite polycrystalline structure is obtained, the films exhibit high transparency again. The upconversion emission intensities under the 970 nm laser excitation are in the same order in the samples annealed at 450, 550 and 650 C., when the films have similar thickness, transparency and surface morphologies. It means that the upconversion efficiency in the nanocomposite structure is as good as, if not better than that of the fully crystallized perovskite structure of PLZT.

[0048] Average sizes of the pyrochlore phase nano-crystallites have been extracted from the XRD data using Scherrer's equation as about 4 nm in the 450 C. annealed samples, The formation of the nano-crystallites embedded in an amorphous phase in the samples annealed at 450 C. has been confirmed by TEM observations. The observed average size of the nano-crystallites by TEM is in accordance with the XRD analyses. The range of the nanoparticle sizes is between 1 nm and 10 nm. It can vary between 1 nm to 300 nm depends on the composition and preparing conditions. Light scattering will not occur in such a nano-composite structure due to very small sizes of the nano-crystallites and lack of clear grain boundaries which cause a large different index change. This results a high transparent material.

[0049] Shown in FIG. 5 is a typical TEM image of the nano-composite, in which the nano-crystallites (arrow positions) embed in the amorphous matrix (rest of the materials). The TEM samples are prepared by scratching off the PLZT:Er, Yb film with a blade. The fine powdery materials collected are further ground and then dispersed in ethanol.

[0050] In a preferred embodiment, nano-composite film is lanthanides doped Pb.sub.1zLa.sub.z(Zr.sub.xTi.sub.1x).sub.1z/4O.sub.3, which is an upconverting material and can convert infrared lights into visible light. Shown in FIG. 6 are upconversion emission spectra of the nano-composite films, which is transparent for visible light.

[0051] Shown in FIG. 7 is a transmission spectrum of the upconverting nano-composite thin film. The thickness of the film is 2 m. It is measured using a Perkin-Elmer (Lambda 9 UV/VIS/NIR) spectrophotometer. Shown in FIG. 8 is a surface AFM (Digital Instrument: Nanoscope IV) image. The surface root square roughness of a 500 nm thick nano-composite film is 0.2 nm, indicating a very smooth surface. The coatings are also very dense. Thus they possess excellent mechanical durability and chemical stability. Cross-sectional microscopies confirmed that the bindings at substrate and film interface are very firm, as shown in FIG. 9. The fractured cross-sectional surface exhibited a seemingly columnar growth pattern and without visible voids.

[0052] Another preferred embodiment is to produce non-crystalline compound powders by heating of a solution mixture of metal salts or metalorganic compounds. The amorphous matrix in a bulk format can be made by mixing and consolidating of these non-crystalline compound powders. Then nano-crystallites can be precipated from the amorphous matrix during heat treatment to form nano-composite structure.

[0053] Yet another preferred embodiment to prepare the non-crystalline compound powders is by ball-milling of corresponding chemicals.

[0054] Although particular embodiments of the invention have been described and illustrated herein, it is recognized that modifications and variations may readily occur to those skilled in the an. For example, other film fabrication technologies can be used to make the structure, or different precursor can be used in a solution coating method.