FIELD OF THE INVENTION
The invention relates to a microwave assisted synthesis method to produce high-
dielectric constant perovskite structured CCTO powder in nano (20-100 nm) and
micro (1-5 μm) form.
More particularly the present invention relates to a microwave assisted synthesis
method to produce high dielectric constant copper calcium titanate (CCTO)
powder in nano and micro form.
BACKGROUND OF THE INVENTION
Owing to the miniaturization of electronic devices, giant dielectric constants
electro-ceramics such as CaCu3Ti4O12 (CCTO) with working range over a wide
range of frequencies and temperatures have been in demand. To a great extent
the processing conditions of CCTO influence the dielectric behaviour. Perovskite
like CCTO powders are prepared by several methods including solution, solid-
state, mechano-chemical methods etc [ 1-3 ]. The traditional solid-state reaction
method for the preparation of CCTO suffers from the disadvantages of non-
uniformity. The salt impurities resulting from the incomplete reaction during the
solid-state reaction may be a reason for it. Mechano-chemical methods require
long milling times (>20 h), or additional high temperature (900-1100°C) heat

treatments with repeated intermediate grindings to produce pure phase.
Compared to the solid- state, mechano-chemical methods, solution synthesis
offer particular advantages in producing powders with smaller particle sizes,
because of homogenous mixing of metal cations and reduced temperatures for
phase formation. Further, the powders prepared by solution methods tend to
have better sintering densities [1-4].
Solution routes to bulk CCTG materials include sol-gel [1], soft chemistry
synthesis [2] and oxalate precursor method [3]. CCTO synthesis by solution
route has also been disclosed in several patents.
In Chinese Patent CN101798213B, synthesis of CCTO is achieved by mixing
CacO3, CuO, TIO2, MgO and SrCO3 according to the required stoichiometric
proportion to obtain a mixture followed by grinding the mixture into powder, and
sequentially carrying out drying, pre-buming and ball milling. The disclosed
process is hard to adapt for industrial scale production.
Japanese Patent JP 2012517955A discloses the synthesis of CCTO by sintering of
precursor powders, which is a tedious and lengthy process.
Chinese Patent CN 101792308B 2012-06-27 discloses the synthesis of shell-core
structure ceramic material by sol-gel method. The disclosed methods reliability is
relatively less as many parameters have to be controlled in sol-gel synthesis and
it is also affected by the environment under which the synthesis is performed.

Chinese Patent CN 102432062A 2012-05-02 discloses the synthesis of CCTO with
nitrates of Cu and Ca as well as Ti (OC4H9)4 as starting materials; the method of
the invention combines a hydrothermal method for preparing nanometer CCTO
powder. However, the precursors are explosive and not suitable for industrial
production.
Chinese Patent CN101671174B 2012—02-22 discloses the preparation of CCTO
from initial raw materials of CaCO3 or CaO, CuO and TiO2, by mix grinding and
then processed by pre-sintering at the temperature from 800 to 950°C after mix
grinding. The disclosed process is hard to adapt for industrial scale production.
USA Patent US 7830644B2 2010-11-09 discloses the synthesis of CCTO by solid
state reaction of powders, which is a tedious and lengthy process.
All the above disclosures or inventions for the synthesis of CCTO either involve
explosive precursors or time consuming solid state synthesis methods or sol-gel
synthesis methods. All of them are relatively time consuming and it is relatively
harder to achieve consistency. So, a method which is relatively easy, one that
uses non-explosive pre-cursors and one that can reproduce the CCTO powder
consistently is required.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a microwave assisted
synthesis methods to produce high-dielectric constant perovskite structured
CCTO powder in nano (20-100 nm) and micro (1-5 μm) form.

Another object of the invention is to propose a microwave assisted synthesis
methods to produce high-dielectric constant perovskite structured CCTO powder
in nano (20-100 nm) and micro (1-5 μm) form which is relatively faster, reliable.
A further object of the invention is to propose a microwave assisted synthesis
methods to produce high-dielectric constant perovskite structured CCTO powder
in nano (20-100 nm) and micro (1-5 μm) form utilizes non-hazardous (i.e.
environmentally benign) and non-explosive precursors.
SUMMARY OF THE INVENTION
A representative embodiment of a method for preparing CCTO powder (nano
(20-100 nm) and micro (1-5 μm)) constitutes an oxalic acid solution precipation
method, which essentially consists of heating non-explosive hydrate/di-hydrate
salts of copper, calcium, and titanium in oxalic acid by microwave irradiation
(120-210°C). The method can produce kilogram quantities of filterable metal (Cu,
Ca and Ti) oxalate precursor. The metal oxalate precursor is thermally
decomposed (calcinations) in atmospheric/vacuum furnace (600-900°C) to form
CCTO. This simple synthetic route produces high yield metal oxalate from
commercially available precursors and is free of pH adjustments. In-addition, the
calcinated powders can be we ball milled (30-90 minutes) and dried in
atmospheric/vacuum over (50- 90°C for 30-90 minutes) to achieve CCTO phase
with relatively uniform grain size.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
1. Figure 1 X-ray diffraction (XRD) of metal oxalate precursor powder before
thermal decomposition.
2. Figure 2 XRD of thermally decomposed CCTO powder
3. Figure 3 XRD of ball milled CCTO powder after thermal decomposition
4. Figure 4 Scanning electron microscopy (SEM) image of CCTO powder.
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention provides for a method to prepare
CCTO powder by heating commercially available precursors by microwave
irradiation. The embodiment of heating precursors by microwave irradiation
can be explained as follows. Among all heating methods, microwave
irradiation is an efficient method that can produce uniform end product.
Microwave energy is considered as a heat source that leads to faster chemical
reactions, better product yield and purity than conventional heating methods.
Further, microwave synthesis, due to its volumetric heating, is unique in its
ability to be scaled up without suffering from thermal gradient effects. It can

potentially be scaled up to produce nanomaterials by either large microwave
systems or by microwave flow reactors [4]. Microwave dielectric heating apart
from improving the material formation rate, also enhances the quality and size
distributions. The reaction rates are influenced by the microwave field and by the
choice of precursors. The final quality of the generated materials depends on the
reactant choice, applied power, reaction time, and temperature.
The precursors utilized for preparing CCTO can include, but are not limited to
any commercially available hydrate/di-hydrate salts of copper, calcium, and
titanium of 90% purity and higher, oxalic acid solution of 0.1-1M. Microwave
systems can include any professional systems from Antonpar or CEM or
Milestone.
Now having discussed various aspect of CCTO synthesis by microwave
irradiation, embodiments on preparation of CCTO follows.
To prepare CCTO first the stoichiometric amount of precursor powders i.e.
hydrate/di-hydrate salts of copper, calcium, and titanium are mixed with oxalic
acid solution. Oxalic acid solution was prepared by adding required amount of
oxalic acid to Dl water. The entire solution of oxalic acid and the metal salts was
transferred into Teflon or Quartz vessels. The sealed vessels are placed in the
microwave cavity. The entire solution is stirred for 10-30 minutes , followed by
heating (120-210°C) with 2.5 GHz microwave irradiation for 30-90 minutes under
constant stirring. The reaction occurring during microwave heating can be
summarized as

CaCl2.2H2O+3CuCl2.2H2O+4TiCl3+H2C2O4+H2O → Metal Oxalate Powder + Other
by-products.
After completion of the reaction, metal oxalate powder is extracted by filtering or
centrifugation. Figure 1 illustrates the XRD pattern of the metal oxalate powder
consisting of CCTO and other minor phases including CuO, CaTiO3 and TiO2 The
metal oxalate powder is thermally decomposed (calcinated) at 600-900°C in
atmospheric/vacuum furnace for 5-10 hours. Thermal decomposition leads to
improvement in the CCTO phase and reduction of the minor phase concentration
which is observed in figure 2. Finally it is preferable to ball mill the thermally
decomposed powder for 30-90 minutes followed by drying in an
atmospheric/vacuum oven for 30-90 minutes at 50-90 °C. Ball milling further
improved the CCTO phase and the particle size which can be seen in figure 3
and 4. All the above processes results in a uniform CCTO phase of even particle
distribution in terms of shape and size. Further the disclosed process allows one
to tune the chemistry of the oxalate precursors and, hence, the chemical
composition of the calcinated precipitation products. The application of CCTO
synthesized by the disclosed method can be in capacitors, filters in polymer
composites etc.
Although various embodiments of this invention have been shown and described,
it should be understood that various modifications and substitutions, as well as
rearrangements and combinations of the preceding embodiments can be made
by those skilled in the art, without departing from novel spirit and scope of
invention.

b) NON - PATENT LITERATURES
1. Dielectric properties of CaCu3Ti4O12 prepared by sol-gel self-combustion
technique, C. Kumar, J Mater Sci: Mater Electron 22 (2011) 579-582.
2. In situ synthesis and characterization of polyaniline-CaCu3Ti4012 nano-
crystal composites, P Thomas et al., , Synthetic Metals 159 (2009) 2128-
2134
3. Soft chemistry synthesis of the perovskite CaCu3Ti4012 Loic Marchin et al.,
Progress in Solid State Chemistry 36 (2007) 151-155
4. Microwave-assisted synthesis of colloidal inorganic nano-crystals,
M.Baghbanzadeh et al., Angewandte Chemie, 50 (2011) 11312-11359.

WE CLAIM
1. A microwave assisted synthesis method to produce high-dielectric
constant perovskite structured CCTO in nano (20-100 nm) and micro (1-5
nm) form comprising the steps of :
- heating (120-210 °C) a hydrate/di-hydrate salts of copper, calcium, and
titanium in oxalic acid by microwave irradiation (2.5GHz) for 30-90
minutes;
- extracting the microwave synthesized metal oxalate precursor powder by
filtration or by centrifugation;
- allowing thermal decomposition of the metal oxalte precursor powder at
600-900°C in atmospheric/vacuum furnace for 5-10 hours;
- wet ball milling of the thermally decomposed powder for 30-90 minutes;
and
- drying the ball milled powder in an atmospheric/vacuum oven for 30-90
minutes at 50-90°C.

2. The method as claimed in claim 1, wherein the size of CCTO powder in
claim 1 can vary from 20 nm to 5 μm.
3. The method as claimed in claim 1, wherein the synthesis temperature and
time by microwave irradiation range from 120-210 °C and 30-90 minutes.

4. The method as claimed in claim 1, wherein concentration of oxalic acid
varies from 0.1-1M.
5. The method as claimed in any of the preceding claims wherein the
precursor salt is a commercially available hydrate/di-hydrate salts of
copper, calcium, and titanium.
6. The method as claimed in any of the preceding claims, wherein the
oxalate powder is thermally decomposed either in open air or inert
atmosphere or vacuum, decomposition temperature and time varying
from 600-900°C and 5 to 10 hours.
7. The method as claimed in any of the preceding claims, wherein the
solution used for ball milling is a commercially available acetone, ethanol
or iso-propanol, wherein the ball milling time varies from 30-90 minutes.
8. The method as claimed in any of the preceding claims wherein the drying
atmosphere after the ball milling can be open air or vacuum or inert, and
wherein the drying temperature and time varying from 50-90°C and 30-90
minutes.

ABSTRACT

The present invention discloses a microwave assisted synthesis method to
prepare nano (20-100 nm) and micro (1-5) μm copper calcium titanate
(CCTO) powder. CCTO synthesis is performed by an oxalic acid solution
precipitation method, which includes microwave heating (120-210 °C) of non-
explosive, hydrate/di-hydrate salts of copper, calcium, and titanium in oxalic
acid. The method is enabled to produce kilogram quantities of filterable metal
(Cu, Ca and Ti) oxalate precursor. According to the invention the metal
oxalate precursor is thermally decomposed (calcinations) in
atmospheric/vacuum furnace (600-900 °C) and wet ball milled and dried in
atmospheric/vacuum oven (50-90 °C) to form CCTO. Advantages of the
disclosed CCTO preparation are multi-fold, one is non-explosive and
environmentally benign precursors and second the reliability and repeatability
of the microwave process appropriate for manufacturing that can meet the
industrial application.