FORM 2
THE PATENTS ACT, 1970
(39 Of 1970)
As amended by the Patents (Amendment) Act, 2005
&
The Patents Rules, 2003
As amended by the Patents (Amendment) Rules, 2006
COMPLETE SPECIFICATION
(See section 10 and rule 13)
TITLE OF THE INVENTION
A method for auto combustion synthesis of nano ZnO powder
APPLICANTS
Crompton Greaves Limited, CG House, Dr Annie Besant Road, Worli, Mumbai 400 030, Maharashtra, India, an Indian Company
INVENTOR
Roy Pradip, Crompton Greaves Limited, CG Global R&D Centre (AMPTC), R&D Building No 2 (Nagarjuna), Kanjur Marg (East), Mumbai 400042, Maharashtra, India, an Indian national
PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the nature of this invention and the manner in which it is to be performed:

FIELD OF THE INVENTION
The invention relates to a method for auto combustion synthesis of nano ZnO powder. This invention also relates to nano ZnO powder obtained by the method.
BACKGROUND OF THE INVENTION
ZnO has a wurtzite structure characteristic of semiconductors and is extensively used in electronic ceramic devices and photocatalysts. Nano structured zinc oxide has exceptionally good semiconducting, piezoelectric and pyroelectric properties and many important applications in optoelectronics, varistors, nano resonators, nano detectors and biomedical electronics. Nanoscale particles possess different physical and chemical properties as compared to bulk, materials. Better sintering ability and higher catalytic activity are some of the properties of nano particles because of their characteristics like nano sized crystallite, large surface area and different surface properties. Vapour method and sol-gel method are generally used to make nano sized zinc oxide powder. The ZnO obtained by the vapour method are agglomerates rather than nano powders because of the reaction conditions being difficult to control. Besides, the vapour method is.also time and energy consuming. Sol-gel method produces uniform ZnO powder. However, strict control of the reaction condition is necessary but difficult because of the violent nature of the hydrolysis reaction in the air during the synthesis. In addition, this method also suffers from high material cost because of which it is not commercially viable. Preparation of nano sized zinc oxide powder by auto combustion or self-propagating high temperature synthesis (SHS) is known. In one method, zinc nitrate and glycine are used in 1:0.90, 1:1.11 and 1.2.25 molar ratios. The surface area of the particles obtained by
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this method are 20.8, 17.6 and 35.6 respectively. [Mat Sci & Engg B 1ll (2004) 197-206]. These particles are obviously low surface area. In another method zinc nitrate and urea are used in the molar ratio 1:1.67. The calcination of the as-burnt ash formed by auto combustion is carried out at 500°C. Particle size of ZnO obtained is reported to be 389 nm and the surface area of the particles is reported to be 2.75 m2/gm. The particles are reported to be spherical shape. [Int J Inorg Mat 1 (1999) 235-241]. These particles are of high size and low surface area. In another method zinc nitrate and urea are used in 1:1 molar ratio. Particle size is reported to be 4.2nm and the surface area is reported to be 2.753 m2/gm [Ceram Int 26 (2000) 561-564]. These particles are also of high size and low surface area. The above literatures do not describe the other characteristics and details of the ZnO particles and methods.
OBJECTS OF THE INVENTION
An object of the invention is to provide a method for autocombustion synthesis of nano ZnO powder, which method is simple and easy to carry out and it economical.
Another object of the invention is to provide a method for auto combustion synthesis of nano ZnO powder, which method consumes lower amount of external energy.
Another object of the invention is to provide a method for auto combustion synthesis of nano ZnO powder, which method produces homogeneous and highly reactive zinc oxide powder.
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Another object of the invention is to provide nano ZnO powder obtained by the method. DETAILED DESCRIPTION OF THE INVENTION
According to the invention there is provided a method for autocombustion synthesis of nano ZnO powder which comprises the following steps:
i) mixing aqueous solutions of zinc nitrate and citric acid in the molar ratio
1:1;
ii) heating the mixture at 80°C to obtain a gel;
iii) allowing auto combustion of the gel to obtain a fluffy loose powder as-burnt ash;and
iv) calcining the as-burnt ash at 400°C.
According to the invention there is also provided a nano ZnO powder having average crystallite size 3.42nm, average particle size 40-50nm, average powder density 3.01 gm/cc, average surface area 60m2 /gm and average spherical shape.
The following experimental example is illustrative of the invention but not limitative of the scope thereof.
Example 1 Solutions of zinc nitrate (73.08 gm) and citric acid ( 5I.63gm) in deionized water were mixed in 1:1 molar ratio. The aqueous mixture was heated at 80°C for 10 hrs to obtain a gel. The auto combustion of the gel was allowed to take place for 2 hrs until the gel was completely burnt out to form a fluffy loose powder as-burnt ash. The auto ignition of the gel was carried out in a glass beaker upon an electric hot plate. The as - burnt ash was
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Average crystallite size Average surface area Average particle size Average shape Average powder density
calcined at 400°C for two hours to obtain a homogeneous powder (20 gm). Crystallite size of the powder was calculated from peak broadening using Scherrer formula (Dhk1 = 0.9 X pCos 9). BET specific surface area of the powder was determined using a Quantachrome Quantasorb apparatus. Particle size and shape were calculated by particle size analysis and TEM (Transmisson electron microscopy). Density was determined by powder density measurement method. The measured values were the following:
3.42nm
60m2 /gm 40-50nm Spherical and 3.01 gm/cc
It is seen from the above experimental values that the zinc oxide powder obtained by the invention had very low average particle size and very high surface area. Therefore, the powder was highly reactive and very useful in the applications discussed earlier in this specification. The calcination temperature of the as-burnt ash was low correspondingly reducing the energy requirement of the invention. The method of the invention is also simple and easy to carry out. The reactants used in the method of the invention are also inexpensive thereby rendering the invention further economical.
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We claim:
i. A method for autocombustion synthesis of nano ZnO powder which comprises
the following steps:
i) mixing aqueous solutions of zinc nitrate and citric acid in the molar ratio
1:1; ii) heating the mixture at 80°C to obtain a gel;
iii) allowing auto combustion of the gel to obtain a fluffy loose powder as-burnt ash; and iv) calcining the as-burnt ash at 400°C.
2. Nano zinc oxide powder having average crystallite size 3.42nm, average particle
size 40-50nm, average powder density 3.01 gm/cc, average surface area 60m2 /gm and average spherical shape.
Dated this 31st day of July 2008.

(Jose M A)
of Khaitan & Co
Agent for the Applicants
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