|Title of Invention||
AN IMPROVED ENVIRONMENT FRIENDLY, ENERGY SAVING MICROWAVE PROCESS FOR FAST SINTERING OF SHAPED PORCELAIN COMPONENTS IN AIR
|Abstract||This invention relates to an improved environ-friendly, energy saving microwave process for fast sintering of shaped porcelain components in air from quartz based raw materials consisting of feldspar, washed clay, raw clay, quartz, pyrophyllite and felcite comprising: firing shaped component in a microwave furnace using a suitable casket and susceptor arrangement in air at peak sintering temperature of 1200°C to 1260°C with soaking period of 30 minutes; maintaining heating cycle from 6 to 10 hour during sintering; resulting densified porcelain product with reduced porosity, higher bending strength and electrical properties through phase formation comprising of Quartz, Mullite and amorphous.|
FIELP OF THE INVENTION
The present invention relates to an improved, fast, ener y saving environment
friendly microwave process for the firing of porcelain components. The method
is highly energy efficient and can be used in a commercial scale to fire porcelain
components in air. The said method achieves better properties of the product
than its conventional processed counterparts.
BACKGROUND OF THE INVENTION
Porcelain has been used as an electrical insulating material for more than 150
years. For a long time, it has been realized that several characteristic properties
of porcelain (e.g. mechanical strength, high-power dielectric strength and
corrosion resistance) as a ceramic product cannot be obtained in other materials.
Porcelain is distinguished by the lack of open porosity in fired body. This ceramic
is commonly referred as a triaxial whiteware primarily composed of clay, flux and
filler. Potash feldspar or sodium feldspar is generally used as fluxes and quartz
or alumina is used as fillers depending on their end use. Two types of porcelain
insulators are used, the silica and alumina porcelain. In the disclosure these
materials are referred as quartz body and alumina body porcelain.
In quartz based porcelain, high temperature or long firing time leads to a
reduction of solid quartz content in ceramic body because of melting of quartz
grains. This reduction causes the decrease of mechanical strength of porcelain.
Further, the difference between thermal expansion corresponding to quartz
grains and the surrounding liquid phase causes mechanical stress, which can
produce microcracks in porcelain. Intense changes of piece temperature could
lead to increase in the already existing microcracks, causing reduction of
mechanical strength under load. In alumina porcelain, the major portion of
quartz is replaced by alumina, which leads to increase in mechanical strength
(related to smaller number of cracks). During sintering, mullite and corundum
are formed and porcelain is obtained with high content of glass phase that leads
to non-porosity but without the melting of alumina grains (thus high temperature
and long time of firing do not affect the mechanical strength). The alumina
porcelain is insensitive to temperature changes and mechanical strength is
mainly controlled by quantity of corundum (and not by the amount of mullite as
in silica porcelain.
The manufacturing of porcelain insulator involves mainly three steps: forming,
drying, and firing. After forming they are dried in order to achieve the critical
moisture level, below which there will not be further shrinkage of the porcelain
body during drying. Ceramic processing is based on the sintering of powder
compacts rather than melting / solidification / cold working (characteristic for
metals). Sintering commonly refers to processes involved in the heat treatment
of powder compacts at elevated temperatures, usually at T > 0.5 Tm, in the
temperature range where diffusional mass transport is appreciable. Successful
sintering usually results in a dense polycrystalline solid. During sintering this
triaxial composition undergoes varied chemical reactions at different
temperature. Fully sintered porcelain consists of a glassy matrix with quartz or
alumina crystals with primary or secondary mullite, which is responsible for the
strength of the fired porcelain.
In regular practice, insulator is fired in the range of 1200° C to 1300° C with a
cycle time exceeding 100 hrs. Therefore, the porcelain insulator manufacturing
process is an energy and time intensive process, which will take 14 - 20 days of
time and enormous amount of energy. The large proportion of the selling price
of these items represents energy costs. While ceramic producers have always
had a high awareness of energy costs, they are increasingly motivated to act, as
energy prices increase and global competition intensifies. More than 70% of
energy in ceramics manufacturing is accounted for the drying and firing process,
with oil and gas as the predominant fuels.
To solve this problem, an alternative method is adopted with microwave heating.
This is a most promising technology over the conventional one with an
advantage of time and energy savings. The process enables high rate of heating
resulting in cycle time reduction and low energy consumption. This facilitates
low cost of the product. Further, materials will experience the minimization of
grain growth leading to improved mechanical strength. The volumetric heating
will avoid any temperature gradient inside the body. The process is based on
clean electrical energy and thus is an environment friendly technology.
The microwave processing of porcelain is rather complicated since porcelain as
such is a complex material and the fast heating rate by microwave processing
poses further challenge in achieving the proper phase formation. The reactions
occurred during firing needs to be properly controlled in order to achieve the
best properties. The additional reducing atmosphere during processing
complicates the overall processing. Further, the earlier experiments have not
used the commercial compositions, rather concentrated on laboratory level-
experiments, which is a problem in scale-up for commercial exploitation.
DESCRIPTION OF THE INVENTION
One of the objective of the present invention is to provide an improved process
for the firing of an indigenously available commercial quartz based porcelain
material in air.
Another objective of the invention is to reduce the firing cycle of electro
porcelain materials in the range of 75 - 85 % of that employed in the present
commercial conventional process.
A further objective of the invention is to reduce the energy requirement of
processing porcelain materials in the range of 60 - 80 % of that used in the
conventional commercial process.
A still another objective of the invention is to confirm the retention of the
required properties of the porcelain insulator body after processing by a very
rapid firing cycle.
PRIOR STATE OF THE ART
Porcelain is a complex material and sintering to full density with required phases
need careful consideration. This is the main reason of not much literature
available on the microwave sintering of porcelain materials.
S. Takeyama, M. Mizuno, S. Obata, T. Shimade, K. Satake, M. Sato, T. Mutoh, S.
Ito, K. Ide, T. Inoue, K. Asaki, 0. Motojima and M. Fujiwara in sintering of
traditional ceramics by Microwaves (84 GHz and 2.45 GHz) and in microwave:
Theory and application in materials Processing V, 305-312, (2001) have reported
to have sintered clay-quartz-feldspar system both by conventional (electric
furnace) and microwave using 2.45 GHz frequency in air. The authors reported
that to achieve the same degree of water absorption, the temperature applied in
microwave hybrid sintering was slightly higher (approximately 20° C) than those
used in conventional firing. However, this result is mainly concentrated on small
1 inch sample pieces which cannot be scaled up for any property measurements
like strength, modulus etc. Since, the processing of porcelain is material specific,
it is pertinent to experiment using indigenously available raw materials. Further,
the result demonstrating that microwave requires higher temperature than that
of conventional process cannot be properly explained and that will increase the
cost of the product.
M. Mizuno, T. Takayama, S. Obata, T. Hirai, T. Shimada, K. Satake, M. Sato, T.
Mutoh, T. Shimotsuma, S. Ito, T. Inoue, K. Esaki, 0. Motojima in analysis of
microwave sintered porcelain, and in microwave: Theory and application in
materials processing V, 313-319, (2001) have reported to have investigated the
effect of 84 GHZ microwave heating on the sintering properties of porcelain
bodies in Kaolin-feldspar-quartz (KFQ) systems which is similar to the quartz
body porcelain material. The study highlighted that it is very important to
consider the temperature difference between liquid amorphous phase and the
solid crystalline phases during microwave processing of porcelain bodies.
However the effect of microwave radiation at widely used 2.45 GHz frequency on
the sintering of these materials is needed to be studied in this material, since 84
GHZ frequency is not used commercially to exploit processing of materials.
Further, the porcelain after firing was porous and thus cannot be used in high
voltage porcelain applications.
SUMMARY OF THE INVENTION
To achieve the objects, and purpose of present invention, a method has been
developed which allows sintering of porcelain components in air using an environment-
friendly process. The processing has been carried out using a microwave furnace
operating at 2.45 GHz frequency. This method enables reduction of cycle time by 70-
80% and reduction in energy cost by 60-80% over conventional method. Further, the
properties of the sintered products as measured by different analytical techniques were
shown to be either comparable or in some cases better than their conventional
counterparts. The process is environment friendly since it does not use the ever-
depleting fossil fuels for processing. The technology developed here can be scaled up in
a commercial scale in processing porcelains for high voltage applications.
According to the present invention there is provided an improved environ-friendly
energy saving microwave process for fast sintering of shaped porcelain components in
air from quartz based raw materials consisting of feldspar, washed clay, raw clay,
quartz based raw materials consisting of feldspar (11-13%), washed clay (20-25%),
raw clay (25-35%), quartz (20-23%), pyrophyllite (8-10%) and felcite (5-8%)
comprising firing shaped component in a microwave furnace using a suitable casket and
susceptor arrangement in air at peak sintering temperature of 1200°C to 1260°C with
soaking period of 300 minutes; maintaining heating cycle from 6 to 10 hour during
sintering; resulting densified porcelain product with reduced porosity, higher bending
strength and electrical properties through phase formation comprising of Quartz, Mullite
The present invention will be better understood by the narrated description for
an embodiment below with reference to the accompanying drawings in which
Figure 1 (a) represents an arrangement of samples for firing and
circular disc sample (b) for temperature measurement.
Figure 2 represents a firing cycle of microwave processing
according to the invention and conventional processing of
porcelain insulator components.
Figure 3 represents X-ray diffraction patterns of quartz body porcelains
which are sintered by conventional and by improved microwave
process, showing the main crystalline peaks for quartz and mullite.
Figure 4 shows scanning electron micrograph of (a) microwave
and (b) conventionally sintered (1240° C) with magnification
of 10,000 for microwave heating time-8 hours against
conventional heating time ~ 90 hours.
The present invention provides an improved method for the microwave firing of
commercial grade and indigenously available quartz based porcelain insulator
components with following raw materials: Feldspar, washed clay, Raw clay,
Quarts, Pyrophyllite and Felcite.
In a conventional commercial process, the raw materials are intimately mixed
with wet mixing in a ball mill followed by filter pressing to obtain the cakes of
desired hardness. These cakes were used for further shaping of actual porcelain
insulators. The components are dried and sintered in a kiln using fossil fuels
with total cycle time exceeding 100 hour in a partial reducing atmosphere.
In the improved method, the shaped components of high (length / diameter ratio
(10: 1) were used for firing in air in a microwave furnace using a suitable casket
and susceptor arrangement. Figure 1 depicts a schematic of samples placed in a
microwave furnace for sintering. The temperature was measured by emissivity
corrected infrared contact less pyrometer. A typical firing curve used in the
present method is depicted in Figure 2 implying drastic reduction in firing cycle
than that employed in the conventional process. The advantage of this method
is that the microwave power can be switched off after the soaking period at the
peak temperature is over, which is not the case in the conventional method. The
peak sintering temperature and the process cycle have been varied to optimize
the microwave firing of porcelain components.
The x-ray diffraction pattern of the sintered product is compared with that
obtained by conventional method (Figure 3) of sintering in a Kiln using fossil fuel.
All the required phases could be fully formed by the very fast method employed
in the present invention, thus necessitating the need to use this technology in a
commercial scale. The microstructure of the product after firing by the new
method also confirmed the phase formation from the identical microstructure as
obtained in the conventional method (Figure 4). Similar microstructure in both
the processes indicates that reducing atmosphere in conventional processing is
not an essential step in developing the right microstructure during fast-fired
microwave processing. From this observation the processing cycle of quartz-
based porcelain has been developed for commercial scale production of porcelain
The invention is illustrated with testing results wherein
Table 1 shows properties of conventional and microwave sintered
cylindrical rods of Quartz based Porcelain insulator components.
Table 2 shows porosity variation along the length of rod
at different conditions and
Table 3 shows phase content of porcelain body after processing
by conventional and the present improved microwave
Table 1 summarizes the best results achieved on the microwave fired
porcelain products indicating better mechanical properties from the
new method and comparable electrical properties. The electrical
properties of microwave sintered porcelain material is improved
which was not reported in earlier literature and confirms that
microwave firing of porcelain is ideal in an industrial scale to
achieve the competitiveness of the product in the market.
The samples were initially fired at 1210° C with a heating cycle of 6 hour and a
soaking period of 30 minute at the peak temperature. It was observed that, the
samples were not fully sintered and the difference in porosity content was very
high from top to bottom of the sample (table 2). The cycle time and
temperature are two most important parameters to achieve uniformed fully
sintered body. Few more experiments were carried out by varying the heating
cycle and the peak temperature keeping the soaking period of 30 minutes
constant in all the experiments. It was observed that, the heating cycle of ~ 8
hours and a peak temperature of ~ 1240° C is ideal in both reducing the porosity
variation throughout the sample and simultaneously densifying the component
uniformly. This series of experiments have confirmed that microwave heating of
these compositions do not require higher temperature than that used in
The peak temperature, duration at peak temperature and the heating cycle are
three important parameters for proper densification of samples. The peak
temperature plays a very important role in final densification of the product and
imparting properties to the final product. In this experiment, the peak
temperature was varied from 1220° C, 1240° C and 1260° C keeping the soaking
duration constant at 30 minutes. All other experimental conditions were kept
constant. It was observed that the bending strength of the samples varied
drastically from 85 MPa for 1220° C firing to 118 MPa for 1240° C firing to 107
MPa for 1260° C firing. This result indicated that the peak temperature of 1240°
C is ideal in imparting maximum strength in this porcelain composition. It Is
pertinent to mention that the same peak temperature is commonly used in the
conventional commercial process, however, the strength of the microwave-
sintered body is significantly higher than that of its conventional counterpart.
Further, this result has established that higher sintering temperature than that
used in commercial process is not suitable for achieving better properties in
The amorphous phase content in porcelain plays an important role in optimizing
the properties. The higher peak temperature and higher heating time are
directly proportional to higher glass phase formation which translates to lower
mechanical properties. Besides the amorphous content, the amount of
crystalline phase like quartz and mullite and cristobalite influences the properties
of porcelain material. It is always desirable to possess lower amorphorous
content and higher crystalline content to obtain better properties in a quartz
based porcelain material. This is limited in conventional processing due to high
heating time. The present method of microwave processing is a very fast
process and it contributes to the phase formation with significant improvement of
The quantitative x-ray diffraction (XRD) analysis was carried out on the porcelain
samples sintered at different conditions and analyzed by Rietvield method. The
quantified XRD results (table 3) on microwave-sintered rods (MR) were
compared with that of conventionally processed rod sample. It was observed
that the amorphous phase content was lower and the crystalline phase content
was higher in the present microwave sintered samples in contrast to that
processed by conventional process. This result is supportive of higher
mechanical properties in these materials as described earlier. The higher quartz
content in microwave-processed samples contributes significantly to the
mechanical properties. The significant amount of cristobalite phase as present in
conventional processed samples was not found in microwave processed samples.
The results have confirmed that the new improved microwave process described
in this invention is ideal for the processing of porcelain materials.
The main advantages of the new improved process are;
a) The improved method is extremely fast process and less energy-intensive
without affecting the properties of the component. This translates into
very low cost for the processing of such materials in an industrial scale.
b) The new method entails the firing of porcelain in air so that a
combination of reducing and oxidizing atmosphere is not required as
employed in the conventional process.
c) The process yields less amorphous phase content and higher crystalline
phase content resulting better properties than that obtained by
conventional commercial process.
d) The electrical properties of the fired product by the new method are
either comparable or better compared to those obtained by the
e) The new method is environment friendly process since it does not utilize
f) The new process does not require higher temperature than that of
The invention as narrated herein with an embodiment and illustrated with test
results should not be read and construed in a restrictive manner as various
adaptations, changes and modifications are possible within the limit and scope of
the invention as defined and encompassed in the appended claims.
1. An improved environ-friendly energy saving microwave process for fast sintering
of shaped porcelain components in air from quartz based raw materials
consisting of feldspar (11-13%), washed clay (20-25%), raw clay (25-35%),
quartz (20-23%), pyrophyllite (8-10%) and felcite (5-8%) comprising:
processing the firing for sintering by electricity for avoiding the use of already
depleting fossil fuels;
firing shaped component in a microwave furnace using a suitable casket and
susceptor arrangement in air at peak sintering temperature of 1200°C to 1260°C
with soaking period of 30 minutes;
maintaining heating cycle from 6 to 10 hour during sintering;
characterised in that,
densified porcelain product with reduced porosity, higher bending strength and
electrical properties is resulted through phase formation of 43.8% of Quartz,
1.3% of Mullite and 54.9% amorphous in microwave heating cycle of 7.5 hour,
peak sintering temperature of 1240°C and soaking time of 30 minutes wherein
reduction in amorphous phase content and increase in crystalline phase content
results better properties of the sintered products.
2. A microwave process for fast sintering of shaped porcelain as claimed in claim 1
wherein the processing requires electricity for firing and thus avoids the use of
already depleting fossil fuels.
3. A microwave process for fast sintering of shaped porcelain components
as claimed in claim 1, wherein phase formation of Quratz, Mullite and
amorphous are respectively of 43.8 %, 1.3 % and 54.9 % at microwave
heating cycle of 7.5 hour, peak sintering temperature of 1240° C and
soaking time of 30 minutes.
4. A microwave process for fast sintering as claimed in the preceding claims
wherein the bending strength of the sintered samples are in the range of
85 MPa to 118 MPa in the firing peak temperature range of 1220° C to
1260° C, the optimized strength being achieved at 1240° C peak
5. A microwave process for fast sintering as claimed in the preceding
claims, wherein lower amorphous content and higher crystalline phase
(Quartz and Mullite) are formed in the microwave heating translating
higher strength than that of the conventional heating with fossil fuel with
high heating cycle of 80-90 hours, in which higher glass phase are
formed translating lower mechanical properties.
6. A microwave process for fast sintering as claimed in the preceding claims
wherein, in the microwave heating combination of reducing and oxidizing
atmosphere is not required as employed in the conventional process.
7. A microwave process of fast sintering as claimed in the preceeding
claims, wherein the microwave power is switched off after the soaking
period of 30 minutes at the peak sintering temperature is over.
8. A microwave process of fast sintering as claimed in the preceeding
claims, wherein in higher dielectric constant and dielectric strength are
achieved than the conventional heating cycle of sintering porcelain
compact indicating higher insulating property achievement from the
microwave heating cycle than that of the conventional heating cycle.
9. A microwave process for fast sintering as claimed in the preceding claims
wherein the energy requirement is drastically reduced due to drastic
reduction in firing cycle resulting cost-effectiveness during production of
10. An improved environ-friendly, energy saving microwave process for fast
sintering of shaped porcelain components as herein described and
AN IMPROVED ENVIRONMENT FRIENDLY. ENERGY SAVING MICROWAVE PROCESS
FOR FAST SINTERING OF SHAPED PORCELAIN COMPONENTS IN AIR
This invention relates to an improved environ-friendly, energy saving microwave
process for fast sintering of shaped porcelain components in air from quartz based raw
materials consisting of feldspar, washed clay, raw clay, quartz, pyrophyllite and felcite
comprising: firing shaped component in a microwave furnace using a suitable casket
and susceptor arrangement in air at peak sintering temperature of 1200°C to 1260°C
with soaking period of 30 minutes; maintaining heating cycle from 6 to 10 hour during
sintering; resulting densified porcelain product with reduced porosity, higher bending
strength and electrical properties through phase formation comprising of Quartz, Mullite
|Indian Patent Application Number||19/KOL/2007|
|PG Journal Number||09/2013|
|Date of Filing||05-Jan-2007|
|Name of Patentee||BHARAT HEAVY ELECTRICALS LIMITED|
|Applicant Address||REGIONAL OPERATIONS DIVISION (ROD),PLOT NO:9/1 DJBLOCK 3rd FLOOR,KARUNAMOYEE,SALTLAKE CITY KOLKATA-700091 ,HAVING ITS REGISTERED OFFICE AT BHEL HOUSE,SIRI FORT,NEW DELHI-110049, INDIA|
|PCT International Classification Number||B32B15/02|
|PCT International Application Number||N/A|
|PCT International Filing date|