Title of Invention

"A PROCESS FOR MANUFACTURING NANO-STRUCTURED ZINC OXIDE TETRAPODS"

Abstract

In the present invention there is provided a process for manufacturing nano-structured zinc oxide tetrapods, wherein pre-cleaned pure zinc granules are subjected to heating in a pre-heated muffle furnace maintained at a temperature in the range of 940 to 980 °C for a period in the range of 4 to 6 minutes. The evaporated zinc in vapour phase reacts with the atmospheric oxygen present in the said muffle furnace to form zinc oxide. This zinc oxide in vapour phase is allowed to deposit on a pre-cleaned non-reactive substrate, which can with stand the furnace temperature. The deposit on the said substrate is a fluffy white material of nano-structured zinc oxide tetrapods. In the process of the present invention nano-structured zinc oxide tetrapods have been produced by thermal evaporation under atmospheric conditions.

Full Text

The present invention relates to a process for manufacturing nano-structured zinc oxide tetrapods. The present invention particularly relates to a process for the growth of nano-structured zinc oxide tetrapod whiskers. The present invention more particularly relates to a novel process of thermal evaporation technique which produces tetrapod structures that consist of a central body and four needle-shaped projections extending from the center. The tetrapods are single crystals with hexagonal, wurtzite type lattice (a = 0.32 nm, c = 0.52 nm). The sharp-tips of tetrapods with high aspect ratio appearing as nanowires are considered as the prospective candidates for electron field emission devices. The large band gap of the zinc oxide makes these nano-scaled objects very useful for small size short wavelength lasers and blue light emitting diodes. Further, nano-structured zinc oxide tetrapods find wide industrial usage such as in paint, polymer and composite manufacturing industries. Nanostructures like nanotubes, nanowires, nanobelts, terapods are the basis of nanoscience and technology and functional components for future electronic, optical, and optoelectronic nanodevices (X.F.Duan et al.. Nature 409, 66, 2001) . Benefits to sizing down to nanoscale is the achievement of superior mechanical properties due to the reduction of number and size of defects that cannot be attained in bulk states. Nanostructures can elastically accommodate a high level of strain allowing remarkable structural flexibility and very high strength upto several tens of Gigapascals (GPa) (J.E. Gordon, The new science of strong materials. 1976, London: Pitman). Nanostructured semi-conducting materials have stimulated immense application due to their extraordinary properties. Among these, the Zinc oxide has attracted significantly due to its wide band gap (3.37eV) and large exciton binding energy (60 meV) for many technological usages. Zinc oxide is a versatile, direct gap II - IV semiconductor. Zinc oxide can give various nanostructures like tetrapod nanorods, nanotubes, nanocombs, nanowires and nanoribbon junction arrays. Improved mechanical properties together with inherent slenderness (high aspect ratio) and its semiconductor like characteristics would be ideal for the application of Zinc oxide nanowires to the atomic force microscope (AFM) probes, especially for measuring the surfaces with higher degree of undulation. Further, high hardness of Zinc oxide will protect the probe tip from wear and damage during operation. Zinc oxide has potential for the widest application; it is a unique material that exhibits semiconducting, piezoelectric, and pyroelectric multiple properties. Using different techniques, nanocombs, nanorings, nanohelixs / nanosprings, nanobows, nanobelts, nanowires, and nanocages of Zinc oxide have been synthesized under specific growth conditions. These unique nanostructures unambiguously demonstrate that Zinc oxide is probably the richest family of nanostructures among all materials, both in structures and properties. The nanostructures could have novel applications in optoelectronics, sensors, transducers and biomedical sciences because it is biosafe (Z. L. Wang, Elsevier Ltd. 2004). It has been investigated by H. Q. Li et al. ( Appl. Phys. Lett. 84, 2004 ) that the current increases linearly with the bias and the conductance jumps upon ultraviolet illumination. The rate of increase upon the illumination is much faster than the decrease rate as the light is off. The decrease rate under vacuum is slower as compared to that in air. These phenomena are related to the surface oxygen species and are further confirmed by in situ current-voltage measurements as a function of oxygen pressure at room temperature. Also, the conductance increases greatly as the temperature is raised. These results demonstrate the dominance of surface oxygen species in the transport process through individual Zinc oxide nanowires, which indicates their potential application to room temperature gas sensors. Strong microwave absorption has been observed in X band and the maximum absorption is enhanced as the concentration of the nanowires increases in the composites (Y. J. Chen et al., Appl. Phys. Lett. 84, 3367, 2004). The low complex permitivity and the low dissipation of the pure nanowires demonstrate the fact that pure nanowires are low - loss materials for microwave absorption in X band. The strong absorption is related to interfacial multipoles at the interface between the polyester and the Zinc oxide nanowires. The zinc oxide nanofibers showed cauliflower-like, disordered, vertically and horizontally aligned morphologies in different temperature regions (C. X. Xu et al. J. Appl. Phys. 95, 661, 2004). The aligned nanofibers were composed of hexagonal zinc oxide with good crystallinity. The field emission of the vertically aligned zinc oxide fiber array showed a low field emission threshold, high current density, rapid surge and high field enhancement factor. The threshold electric field is about 2.4 V/um at a current density of 0.1 µA/cm-2. The field enhancement factor is 299. The interrelationships between the green 510 nm emission, the free-carrier concentration and the paramagnetic oxygen - vacancy density in commercial Zinc oxide phosphors by combining photoluminescence, optical - absorption and electron - paramagnetic - resonance spectroscopies have been reported (K. Vanheusden et al. , J. Appl. Phys. 79, 7983, 1996). It was found that the green emission intensity is strongly influenced by free - carrier depletion at the particle surface, particularly for small particles and for low doping. Our data suggest that the singly ionized oxygen vacancy is responsible for the green emission in Zinc oxide; this emission results from the recombination of a photogenerated hole with the singly ionized charge state of this defect. The hydrogen storage characteristics of the synthesized Zinc oxide nanowires are investigated at room temperature (Q. Wan et al. Appl. Phys. Lett. 84, 124, 2004). The highest storage capacity of 0.83 wt% is achieved under the pressure of about 3.03 MPa, and about 71% of the stored hydrogen can be released under ambient pressure at room temperature. A strong ultraviolet emission at 381 nm is observed. These Zinc oxide nanowires show excellent field emission properties with turn-on field of 0.83 V/um and corresponding current density of 25 µA/cm2 (Y. L. Seu et al. J. Appl. Phys., 95, 3711, 2004). The emitted current density of the Zinc oxide nanowires is 1.52 mA/cm2 at a bias field of 8.5 V/um. The large field emission area factor, ß arising from the morphology of the nanowire field emitter, is partly responsible for the good emission characteristics. The Zinc oxide nanowires having high emission current density and low turn - on field are expected to be used in field emission flat panel display. As regards the physical and crystallographic properties of Zinc oxide, it has a crystal structure of Wurtzite hexagonal (space group: C6mc) with the lattice constants; a = 0.32 nm and c = 0.52 nm. Zinc oxide is composed of a number of alternating planes of tetrahedrally coordinated O2- and Zn2+ ions, stacked alternately along c - axis of hexagonal unit cell. Tetrahedral coordination in Zinc oxide results in non-central symmetric structure that consequently produces the effects of piezoelectricity and pyroelectricity. The material is white in colour with the density of 5.6 gm / cc and melting point of 1978 °C. In recent years, Zinc oxide has achieved tremendous scientific and technological support due to the formation of nanostructures like, nanorods, nanowires, nanobelts, nanocages, tetrapods, etc. Further there are several patents on Zinc oxide which clearly disclose the importance of this material in nano-scale particularly having tetrapod structures. Reference may be drawn to: WO2004060995/ 2004-07-22, EP0572972/ 1993-12-08, EP0356994/ 1990-03-07, US5171480 / 1992-12-15, EP0364597/ 1990-04-25, JP6024745 /1994-02-01, EP0572972 /1993-12-08, EP0385645/ 1990-09-05, EP0384726 / 1990-08-29, EP0379746 / 1990-08-01, EP0358078 / 1990-03-14. All these materials as described in the patents can be used as probes for scanning tunneling microscopy, second phase reinforcements for conductive coatings, nanocomposites and in photodetectors. There are number of methods, such as, thermal evaporation, chemical vapor deposition, hydrothermal, metalorganic vapour-phase epitaxy for preparing the various forms of nano-structured materials of zinc oxide. Among these techniques, it has been realized that the thermal evaporation is a simple and low cost method for synthesis of zinc oxide nano-structures. Reference may be made to US patent no. 5,091,765, of Yoshinaka, et al., titled: Photoconductive cell with zinc oxide tetrapod crystals; wherein is described a photoconductive cell comprising a photodetector section which consists of an aggregate of tetrapod-like zinc oxide whiskers. Each whisker consists of a core and needle-shaped crystals extending in four different directions from the core. Metal zinc powder was ground in a mortar type grinder in the presence of water and kept standing in water for 3 days. After drying, powder was charged in a crucible made of alumina and heated in a furnace kept at 1000.degree.C. for one hour to obtain tetrapod-like zinc oxide whiskers. The needle-shaped crystals had an average diameter of 8.mu.m at the root portions and an average length from the root to the tip end of 50.mu.m. Most of the whiskers were in the tetrapod-like form in which four needle-shaped crystals extended in four different directions, while some whiskers extended in one-, two- and three-directions. In another patent JP4144995, of Sato Takashige et. al., the mechanism of controlling atmosphere conditions to grow the zinc oxide tetrapod whiskers is very complicated and time consuming. The cost of infrastructure handling needs skilled operator to handle the process. Moreover the infrastructure is costly and sophisticated and requires extra care while operation. In yet another patent JP3150299, of Fujiwara Teruo et. at., the furnace design specific for the preparation of zinc oxide tetrapod whiskers is costly and not simple. Moreover, burning gas is used which is not eco friendly. Further, the furnace needs frequent attention while conducting the operation. Reference may be made to H. J. Yuan et al. Chem. Phys. Lett. 371, 337, 2003, wherein mass production of zinc oxide nano-structures such as nanowires, nanoribbons, and needle-like rods has been achieved by method of thermal evaporation of ZnS powders onto a silicon substrate in the presence of Au catalyst. The substrate used in the experimentation has been Si wafers. They are cleaned for 30 min in acetone solution. The cleaned substrates are then deposited with Au film for about 15 s under 10-1 Torr at 100 V and 20 mA by using E1001 film deposition system. The ZnS powders are placed in the middle of quartz boat, and the treated Si substrates are then placed next to the ZnS and along the downstream side of the flowing argon. The ZnS side of quartz boat is placed at the center of the quartz tube that, is inserted in a horizontal tube furnace. The reaction tube is sealed under vacuum. During the growth process of Zinc oxide nanowires, the 200 seem Ar, as carrier gas, is introduced into the vacuum chamber and the total pressure kept at about 150 Torr to avoid the leakage of some oxide gas into the reaction chamber, ZnS can thus be oxidized in to Zinc oxide molecules. After 30 min of typical deposition for growing zinc oxide nanowires, the samples are cooled down to room temperature. Reference may be made to Dai et al. Adv. Func. Mater. (2003) wherein a schematic of the thermal evaporation process, normally carried out in a tri-temperature zone horizontal furnace composed of alumina tube, rotary pump system, gas supply, is shown. The hitherto known method as described above, needs several cumbersome process parameters such as carrier gas, temperature of thermal gradient in furnace, vacuum, which are required to be optimized before the start of the experiment to manufacture the required size, shape and yield of nano-structured zinc oxide. The main object of the present invention is to provide a process for manufacturing nano-structured zinc oxide tetrapods, which obviates the drawbacks of the hitherto known prior art. Another object of the present invention is to provide a process for the preparation of high yield nano-structured zinc oxide tetrapods. Yet another object of the present invention is to provide a process which is very simple and with minimum number of steps and not involving complicated process control steps of the known prior art as described in literature. In the present invention there is provided a process for manufacturing nano-structured zinc oxide tetrapods, wherein pre-cleaned pure zinc granules are subjected to heating in a pre-heated muffle furnace maintained at a temperature in the range of 940 to 980 °C for a period in the range of 4 to 6 minutes. The evaporated zinc in vapour phase reacts with the atmospheric oxygen present in the said muffle furnace to form zinc oxide. This zinc oxide in vapour phase is allowed to deposit on a pre-cleaned non-reactive substrate, which can with stand the furnace temperature. The deposit on the said substrate is a fluffy white material of nano-structured zinc oxide tetrapods. In the process of the present invention nano-structured zinc oxide tetrapods have been produced by thermal evaporation under atmospheric conditions. Accordingly the present invention provides a process for manufacturing nano-structured zinc oxide tetrapods, which comprises; characterized in subjecting pre-cleaned pure zinc granules to heating for a period in the range of 4 to 6 minutes in a pre-heated muffle furnace maintained at a temperature in the range of 940 to 980 °C, allowing the so formed zinc oxide in vapour phase to deposit on a pre-cleaned non-reactive substrate, allowing the deposit laden substrate to cool naturally inside the furnace to obtain nano-structured zinc oxide tetrapods having dimension of 10 to 20 µm including the four legs of nano needles as fluffy white material. In an embodiment of the present invention, the pure zinc granules are of commercial purity. In another embodiment of the present invention, the pure zinc granules are preferably of purity of atleast 3N (99.9%). In yet another embodiment of the present invention, the pure zinc granules are cleaned in ultrasonic bath in acetone. In still another embodiment of the present invention, the pre-heated muffle furnace is preferably maintained at a temperature in the range of 950 to 960 °C. In still yet another embodiment of the present invention, the pre-heated muffle furnace is allowed to stabilize the selected temperature in the range of 940 to 980 °C for about 30 minutes prior to loading the pre-cleaned zinc granules. In a further embodiment of the present invention, the pre-cleaned non-reactive substrate is capable of withstanding the furnace temperature, such as quartz slide. In a still further embodiment of the present invention, the pre-cleaned zinc granules is contained in a pre-cleaned open-top crucible having a pre-cleaned non-reactive substrate placed over the said crucible top. In a yet further embodiment of the present invention, the pre-cleaned zinc granules contained in the pre-cleaned crucible is loaded in the central heating zone of the pre-heated temperature stabilized muffle furnace under atmospheric conditions. In another embodiment of the present invention, the natural cooling of the deposit laden substrate is enhanced by shifting the crucible substrate combination from the central heating zone towards the extreme end of the muffle opening of the furnace. In still another embodiment of the present invention, the nano-structured zinc oxide tetrapods obtained as fluffy white material is of the order of 90% or higher. In yet another embodiment of the present invention, the nano-structured zinc oxide tetrapods obtained is hexagonal wurtzite zinc oxide crystal structure. In still yet another embodiment of the present invention, the nano-structured zinc oxide tetrapods including the four legs of nano needles are of overall dimensions in the range of 10 to 20 µm. In a further embodiment of. the present invention, the nano needle tips of the nano-structured zinc oxide tetrapods are of curvature in the range of 10 to 50 nm. In a still further embodiment of the present invention, the diameter of the nano needle tips of the nano-structured zinc oxide tetrapods are in the range of 20 to 150 nm. In accordance with this invention and based on the physical characteristics of zinc having melting point: 419 °C, boiling point: 906 °C, density 7.14 gm.cc-1, the best results were obtained at around the furnace temperature of 950 °C. At this temperature the molten zinc becomes volatile and enables efficient formation of zinc vapour in the atmosphere to react with oxygen to form nano-structured zinc oxide tetrapods. During experimentation conducted by us under partial vacuum conditions (controlled oxygen) to obtain tetrapods nano structures, resulted in other morphologies of zinc oxide. A few experiments were also conducted in the presence of a catalyst like tellurium, because it was noticed that tellurium form wire like structures when it was evaporated individually instead of zinc. However, the formation of zinc oxide tetrapods was not successful in the presence of tellurium. In the process of the present invention nano-structured zinc oxide tetrapods have been produced by thermal evaporation under atmospheric conditions. The nanostructured tetrapods of zinc oxide produced have a central body with four needles having conical tips towards the end. The dimension of the tetrapods including the four legs of nano needles was about 10 to 20 µm. The curvature of the tips of nano needles varied between 10 to 50 nm. The diameter of the nano needles was between 20 to 150 nm. In the process of the present invention zinc granules were cleaned in ultrasound bath in acetone to make the granules free of grease and dust. The crucible containing the Zn was inserted in a highly insulated muffle furnace for thermal evaporation. The quartz crucible was then inserted only after the furnace reached the desired stable temperature for half an hour prior to loading the material. The molten zinc evaporated to form zinc in vapor phase which reacted with the atmospheric oxygen to form nano structured zinc oxide. The nano structured zinc oxide was allowed to deposit on a quartz slide placed horizontally at the opening of the crucible. A quartz slide was used as it does not get softened at the working temperature. After 4 to 6 minutes the crucible slide combination was shifted from the central heating zone towards the extreme end of the muffle facing atmospheric condition to further natural cooling. The deposited material so obtained is white spongy or fluffy and is wurtzite hexagonal zinc oxide crystal structure. The novelty of the process of the present invention for manufacturing nanostructured zinc oxide tetrapods, resides in the simplicity of the process wherein yield obtained is very high of the order of more than 90%, without employing any inert conditions. Economy is the supplementary feature of the novelty. The novelty of the process of the present invention for manufacturing nano-structured zinc oxide tetrapods, has been realized by the non-obvious inventive steps wherein bare metal in the form of pre-cleaned pure zinc granules instead of the metal oxide, such as zinc oxide, is subjected to heating in a pre-heated temperature stabilized muffle furnace maintained at a temperature in the range of 940 to 980 °C for a period in the range of 4 to 6 minutes. This enabled to make the liquid zinc, a bit volatile, to evaporate and react with the atmospheric oxygen present in the furnace to give rise to the formation of nucleated zinc oxide in nano-structured tetrapod shapes. Thus enabling production of nano-structured zinc oxide tetrapods of yield greater than 90% by thermal evaporation under atmospheric conditions. The following examples are given by way of illustration of the process of the present invention in actual practice and should not be construed to limit the scope of the present invention. In the experiments as given in the following examples the process of the present invention was used for the preparation of nano-structured zinc oxide tetrapods and the physical features were characterized by using electron microscopes. The electron microscopes used were as follows: A scanning electron microscope (SEM, model LEO 440) operated at the electron accelerating voltage of 15 kV in secondary electron mode and back scattered electron mode was used to study the morphology of the product. An energy dispersive spectrometer (EDS, model Oxford Link ISIS 300) equipped with SEM was used to study the composition of Zn and O at different parts of the tetrapod structure in the product. A transmission electron microscope (TEM, model JEOL JEM 200CX) working at the electron accelerating voltage of 200 kV in the bright field and selected area electron diffraction mode was used for the internal structural analysis at high magnification. Example 1 Nano-structured zinc oxide tetrapods obtained at furnace temperature of 950 °C and heating time of 5 minutes were studied. SEM observations have shown a high yield of the formation of tetrapods in the microstructure. The size of the tetrapods including needles were about ~ 15 µm in size. In figures 1 & 2 of the drawings accompanying this specification is shown the micrographs which depicts (1) high yield of tetrapods and (2) different size and nano-tips of tetrapods. A single tetrapod has been displayed as in figure 3 of the drawings. In the microstructure, the presence of long nanowires of few micro-scale in length along with tetrapods has also been delineated. An energy dispersive spectrum (EDS) as shown in figure 4 of the drawings indicates that the tetrapods and nano-wires are constituted of Zn and O. As measured by using TEM, the curvature of the tips of the nanowires is about 30 nm, as shown in figure 5 of the drawings accompanying this specification. In general the curvature of the tip was observed between 10 to 50 nm. Example 2 Nano - structured zinc oxide tetrapods obtained at furnace temperature of 940 °C and heating time of 4 minutes were studied. The microstructural analysis reveals a substantial amount of wire formation along with high yield of the formation of tetrapods. The size of the tetrapods including needles were about 30 µm in size. In figure 6 of the drawing accompanying this specification is shown the micrographs which depicts high yield of tetrapods along with nanowires Example 3 Nano - structured zinc oxide tetrapods obtained at furnace temperature of 980 °C and heating time of 6 minutes were studied. The microstructural analysis reveals a substantial amount of wire formation along with high yield of the formation of tetrapods. The size of the tetrapods including needles were about 6µm in size. In figure 7 of the drawing accompanying this specification is shown the micrographs which depicts high yield of tetrapods. Example 4 Nano - structured zinc oxide tetrapods obtained at furnace temperature of 980 °C and heating time of 4 minutes were studied. The microstructural analysis reveals a substantial amount of wice formation along with high yield of the formation of tetrapods. The size of the tetrapods including needles were about 4 µm in size. In figure 8 of the drawing accompanying this specification is shown the micrographs which depicts high yield of tetrapods. The main advantages of the process of the present invention are: 1. Simple and economic process. 2. High yield of the product, more than 90 %. 3. An environment friendly non-hazardous process. 4. The deployment of just one person for the operational management of the entire process as the process does not require any skilled, high-tech, man power to run the system during operation and production of the material. 5. No sophisticated equipment is needed, only a temperature controlled muffle furnace, capable of attaining temperature up to 1100 ° C is required. We Claim: 1. A process for manufacturing nano-structured zinc oxide tetrapods, which comprises; characterized in subjecting pre-cleaned pure zinc granules to heating for a period in the range of 4 to 6 minutes in a pre-heated muffle furnace maintained at a temperature in the range of 940 to 980°C, allowing the so formed zinc oxide in vapour phase to deposit on a pre-cleaned non-reactive substrate, allowing the deposit laden substrate to cool naturally inside the furnace to obtain nano-structured zinc oxide tetrapods having dimension of 10 to 20 µm including the four legs of nano needles as fluffy white material. 2. A process as claimed in claim 1, wherein the pure zinc granules are cleaned in ultrasonic bath in acetone. 3. A process as claimed in claims 1-2, wherein the pre-heated muffle furnace is preferably maintained at a temperature in the range of 950 to 960 °C. 4. A process as claimed in claims 1-3, wherein the pre-heated muffle furnace is allowed to stabilize at the selected temperature in the range of 940 to 980 °C for 30 minutes prior to loading the pre-cleaned zinc granules. 5. A process as claimed in claims 1-4, wherein the pre-cleaned non-reactive substrate is quartz slide. 6. A process as claimed in claims 1-5, wherein the yield of nano-structured zinc oxide tetrapods obtained as fluffy white material is 90% or higher. 7. A process as claimed in claims 1-6, wherein the nano-structured zinc oxide tetrapods obtained is hexagonal wurtzite zinc oxide crystal structure.

Documents:

773-del-2005-Abstract (21-10-2011).pdf

773-DEL-2005-Abstract-(19-10-2011).pdf

773-del-2005-Abstract-(22-07-2011).pdf

773-del-2005-abstract.pdf

773-del-2005-Claims (21-10-2011).pdf

773-DEL-2005-Claims-(19-10-2011).pdf

773-del-2005-Claims-(22-07-2011).pdf

773-del-2005-claims.pdf

773-del-2005-Correspodence Others-(22-07-2011).pdf

773-DEL-2005-Correspondence Others-(19-10-2011).pdf

773-del-2005-correspondence-others.pdf

773-del-2005-Description (Complete) (21-10-2011).pdf

773-DEL-2005-Description (Complete)-(19-10-2011).pdf

773-del-2005-Description (Complete)-(22-07-2011).pdf

773-del-2005-description (complete).pdf

773-del-2005-drawings.pdf

773-del-2005-form-1.pdf

773-del-2005-form-18.pdf

773-del-2005-form-2.pdf

773-del-2005-Form-3-(22-07-2011).pdf

773-del-2005-form-3.pdf

773-del-2005-form-5.pdf