FIELD OF INVENTION
The present invention relates to a microwave sintering furnace with multiple
microwave source, a microwave sintering method and devices for loading
components in large volume of an object to be sintered, such as a high alumina
ceramic components by pure microwave heating that require sintering
temperatures in the range 1600 - 1630 degree C.
BACKGROUND OF THE INVENTION
Microwave energy has been in use for over 50 years in a variety of applications,
such as communication, food processing, rubber vulcanization, textile and wood
products, and drying of ceramic powders. Microwave sintering process employs
microwaves to fuse powders into the solids, which produces dense products with
better mechanical properties. The Electro-magnetic energy of the wave is
efficiently converted into thermal energy which produces a grain size in the
finished product which is finer than that produced through traditional sintering.
At present, most microwave ovens in use operate at 2450 MHz, which in terms of
a wavelength is 4.8 inch in air, and further operate at 915 MHz, only for selective
applications. The microwave energy is not effectively coupled to some ceramic
components in the lower temperatures say up to 500°C. It is a well known
practice, that in a process of sintering a sample in a laboratory furnace with
silicon carbide or zirconia or graphite beds to initially heat up, the bed, and
transfer the heat to the samples to reach about 500°C, after which the samples
start absorbing,microwaves effectively to proceed with sintering. However, this
process is not viable and effective in large industrial systems and poses several
bottlenecks in realizing potential advantages of using microwaves to process
ceramics. Additionally there are other technical disadvantages in adapting the
laboratory-proven process for large scale systems.

Microwave sintering in large volume for an industrial process is limited essentially
due to the difficulty in generating uniform field in larger volumes whose linear
dimensions are multiples of wavelengths and in which microwave absorption of
components is disproportional. In addition, the kiln furniture arrangement in
which the components are loaded form an equal proportion of the load thus
reducing the overall effectiveness. The heat loss from the fixture including the
components requires an optimization of the insulation arrangement, which poses
a constraint to ensure a minimal thermal gradient. If the parameter of minimal
thermal gradient is not ensured, not only the microwave power and time to
sinter increases but also variation in properties of the produced components is
noticed. Thus, kiln furniture should be capable of getting heated at the same rate
as of the components and should have sufficient hot MOR for the fixture to have
long cycle life. The reference is made here below particularly to patents that
have utility for industrial pure microwave furnaces.
U.S. Patent No.US-4,963,709 (1990) discloses a microwave sintering system and
method for uniform sintering of large and irregular objects with 28 GHz, 200 kW
Gyrotron microwave source. The part is surrounded by a microwave transparent
bulk insulating material to reduce thermal heat loss and to maintain uniform
temperature. The system can be used to generate a temperature of 1200 degree
C.
US Patent No. US-5,449,887 (1995), to Holcombe et al, have demonstrated
superior microwave transparent thermal insulations for high temperature
microwave sintering operations by using boron nitride coated glassy carbon. This
invention allowed boron carbide powder compacts to be microwave sintered
without allowing the compacts to be contaminated with any yttria thermal
insulation for high temperature microwave sintering operation.

US patent No. US-5,808,282 (1998) described a microwave susceptor bed useful
for microwave sintering of ceramics and composites. The susceptor bed contains
granules of major of alumina susceptor material and minor amount of carbon
as the parting agent either dispersed in the susceptor material or as a coating. A
microwave sintering process using this bed and novel silicon nitride products
produced thereby are described.
US Patent publication US-2003/0111462 Al (2003) to Sato et al, and US-Patent
No. US-6,891,140 B2 (2005) described a sintering furnace in an isothermal
environment by controlling the temperature difference between the interior of
the furnace and the surface of an object to be sintered. This is achieved by
creating an iso thermal boundary enclosing the objects to be sintered. The
boundary consisting of coatings of selective materials which partially absorbs the
microwave and the absorption characteristics being similar to that of the objects,
and hence is capable to achieve similar temperature. This in turn creates an iso
thermal boundary, and thereby heat loss from the components is minimized
leading to an uniform sintering of objects in that volume.
Indian patent IN-235072 discloses a method for uniform sintering in large
volume using microwave in which partially absorbing castables are used as an
enclosure to achieve an uniform sintering. Both US 6891140B2D and IN 235072
is aimed to obtain an inner wall temperature matching to that of the component
surface temperature, which interalia reduces heat loss from the surface of the
component including the temperature gradient resulting in an uniform sintering.
However, the prior art at best describe means for creating an iso thermal
boundary enclosing the components to be sintered, but a very little information
is available regarding the material, method and arrangements relating to the kiln

furniture arrangements for large volume sintering as required in industrial kilns.
Further, some experiments are known to have been carried out at 28 GHZ
microwave frequency and a maximum sintering temperature achieved is 1200
degree C. The susceptors used in the prior art are mainly carbon bed which has
limitations for sintering at high temperature and at ambient atmosphere. It is
well known to the kiln manufactures and users in ceramic industry, oil or gas
fired kilns, the kiln fixture arrangements forms a critical part for efficient
processing. The strength and durability of the fixture and the adapted design
criteria in producing such fixture play a crucial part in realizing an optimum
loading, with durability well over 100 of cycles. It is therefore essential to
propose a microwave processing system for large volume sintering of ceramic
components operable at 2.45 GHZ microwave frequency and capable of
achieving a temperature exceeding 1600 degree, and is enabled to the
isothermal system with effective susceptor materials.
OBJECTS OF INVENTION
It is therefore an object of the present invention to propose a pure microwave
sintering furnace operating at a frequency of 2.45 GHz and capable of generating
uniform microwave heating zone with large volume loading of ceramic
components with reduced cycle time of sintering.
Another object of the invention is to propose a microwave sintering method for
sintering an object with microwave heating.
A still another object of the invention is to propose a kiln furniture arrangement
for large volume loading.

A further object of the invention is to propose an insulation enclosure for
enclosing the kiln furniture arrangement to ensure a minimum thermal gradient
for uniform sintering.
The objects of this invention will be apparent from the ensuring description when
read in conjunction with the accompanying drawings.
SUMMARY OF THE INVENTION
The present inventors have studied various technical aspects such as sintering of
objects in pure microwave kiln, the behavior of various materials that can be
used as a kiln furniture including enclosures, having microwave absorbing
characteristics to selectively limit the temperature gradient between the
component surface and the enclosure, characteristics to withstand fast heating
cycles, higher high temperature strengths, a life of more than 100 cycles. In case
of sintering the object surrounded by insulating material, energy of the
microwaves is absorbed and consumed not only in the object to be sintered but
also in the loading arrangement, microwave absorbing enclosure, and insulating
material, the amount of energy necessary for the sintering process increases
significantly which needs to be optimized, followed by development of suitable
material for the kiln furniture and enclosures. The inventors have studied all
these aspects while arriving at the present invention.
According to one aspect of the invention, there is provided a microwave sintering
furnace comprising a furnace body containing an object to be sintered, means
for generating and introducing microwaves via multiple feeding means into the
furnace body, means of loading large volume objects in layers using a specific
furniture arrangement, means for providing selective kiln furniture material with

controllable transparent or microwave absorbing characteristics, means of
providing insulating enclosure for easy loading and unloading of the components,
means of measuring component temperature, and control means for microwave
power control including the sintering process.
In a second aspect of the invention, there is provided a for measuring the
temperature of the component representing the total load in the furnace, means
for controlling the microwave power based on the temperature measured by the
temperature measuring means and thus controlling the process. The furnace
body preferably includes at least two layers of insulating members with different
temperature grades and disposed outside the space in which the object to be
sintered is placed which optimizes the cost of insulation.
The method of microwave sintering of an object comprising the steps of fixture
arrangement while heating the object with microwaves, and selective partially
absorbing porous materials forming an enclosure to eliminate temperature
gradient from surface of the object to the enclosure. The porous material is
preferably heated with microwaves, together with the object to be sintered.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - is a line diagram showing a microwave sintering system according to
an embodiment of the invention.
Figure 2 - is a sectional view showing an applicator of the system exhibiting a
. high temperature insulation assembly and the loading arrangement
of the product.

DETAILED DESCRIPTION OF THE INVENTION
According to an embodiment of the present invention, as shown in figure-1 a
microwave sintering furnace operating at a frequency of 2.45 GHz for heat
treatment of objects with microwaves being provided. The system comprises a
stainless steel applicator (01) embedded with a plurality of magnetrons (02) to
provide a uniform sintering zone. The microwave is introduced into a cavity
through microwave ports (03) embedded in the walls of the applicator (01) with
Teflon windows. Cooling arrangements are provided on both the walls of the
applicator (01) and the magnetrons (02) using a chilling plant (04). Suitable
sealing arrangement (05) is provided in microwave door (06) with vertical
opening on both sides. A layered structure of high temperature insulating
assembly (07) comprising a plurality of vacuum formed insulation boards (08)
occupies the central portion of the cavity. A plurality of alumina-silicon carbide
composite plates including mullite plates (09) are arranged in a manner to
provide uniform temperature inside the insulation cavity. A the green mass is
placed inside the enclosure in one run with temperature measurement by a non-
contact infra red pyrometer (10) by focusing on actual product (11) with known
emissivity. The heating and cooling activities are carried out by integrating the
microwave source (02) with a programmable controller (12). A maximum
temperature of 1650 degree C can be achieved, with four hours of soaking at
peak temperature in order to test the capability of this furnace. The system has
the capability of online detection of microwave leakage (13) if any, including a
limiting mechanism for movement of the doors of the applicator (01). The
microwave sources are simultaneously controlled by a PLC (14) in coordination
with the programmable controller (12). All the safety interlocks for the MW
System are connected to the furnace controls to ensure all safety conditions are
in place prior to starting of the sintering operation. The mode stirrer (15) inside

the cavity maintains the uniformity of microwave zone during operation. The
applicator (01) is provided with exhaust ports (16) on the top to release volatile
matters during initial stages of sintering and to escape blower air during cooling.
In another embodiment, the alumina based components such as grinding media,
flow beans tubes, tiles etc. can be suitably fired at a temperature exceeding
1600 degree C in such microwave systems. Since alumina is not microwave
absorbing at room temperature, indirect heating is provided with a plurality of
porous susceptors (17) to the products (11) by heating the kiln furniture
assembly upto certain temperature after which alumina interacts directly and
generates volumetric and uniform heating. The sintered products have been
tested and found to be either comparable or superior to the conventional
products. The time required to achieve the peak temperature can be drastically
reduced by this method of heating compared to its conventional counterparts,
resulting in advantages in terms of energy-cost in the tune of 10-20 % including
reduction in cycle time of the overall process by similar amount. The system can
be used to sinter any oxide materials of different shapes and shapes and sizes at
a peak temperature not exceeding 1700 degree C.
The above disclosure of the present invention can be explained in detail with few
suitable examples. The examples and the embodiments of the present invention
are illustrative and not intended to restrict the present invention. The present
invention should not be limited to the details described in the present
specification and can be changed without departing from the scope of the
appended claims and the equivalents thereof.
Example 1
Alumina based grinding media fabricated by isostatic pressing with 50 mm green

diameters were loaded in the kiln fixture. 30 Kgs. of such loads were used for
repeated experiments. The heating time was 8h to reach a peak temperature of
1625 degree C followed by a 0.5h soaking at that temperature. The temperature
measurement was carried out using infra red pyrometer focusing a flat object of
similar composition with known emissivity. The heating cycle was programmed
using a programmable controller capable of multi segment programming. The
controller was connected to PLC of the microwave source and hence the cycle
could be programmed by giving appropriate percentage of power to each
magnetron. The products after sintering yielded >99.5 % of theoretical density
with <0.1% water absorption with superior Vicker's hardness and impact test
compared to that of conventionally processed samples. The density of the
product in each layer was found to be similar indicting eh uniformity of
temperature throughout the entire volume.
Example 2
In another example, alumina tubes of 150 mm long and 20 mm OD and 5 mm ID
were used in 30 Kgs. to load in the above system. The products were heated
similarly to the process described in example 1 at a peak temperature of 1620
degree C and with a soaking of 0.5h. The wall temperature of the applicator was
monitored and found to be below 50 degree C indicating the appropriateness of
the insulating arrangement inside the cavity. After the soaking period, the
magnetrons were put off resulting in energy savings during cooling. Forced
cooling can be carried out after 700 degree C by blowing air in the cavity with
the provisions available in the system. The mode stirrers were employed inside
the cavity to maintain uniformity of microwave radiation inside the cavity. The
products after firing displayed > 99.5 % densification with very low water
absorption values.

Example 3
Alumina based tiles (both bend and straight configuration) of 40 Kgs. were
loaded in the microwave chamber and fired at 1600 degree C for 2h with a
heating cycle of lOh against the conventional heating cycle of 16h. The
emissivity of these dark colored alumina compositions as adjusted by proper
calibration inside the microwave cavity and accurate temperature could be
measured using infra red pyrometer in the range of 500-2000 degree C. The bulk
density of the tiles was > 99.5 % of theoretical density with <0.05 % of water
absorption in all four levels of tiles fired in the microwave furnace. Further, a tile
of 150 mm long, 30 mm thick and 75mm width each weighting ~ 550 g was cut
into eight pieces and measured density and water absorption in each segment.
The result of each segment was in agreement with the bulk result of the tiles
confirming the uniformity of the temperature inside the microwave furnace
resulting in uniform densification of the products.
As described above, the sintering furnace and the manufacturing method of
sintered object according to the present invention are useful for manufacturing
the sintered objects by sintering objects to be sintered made of fine ceramic
materials, and are particularly suitable for executing not only a single set point
heating but also a multi segment heating-soaking ramp in the microwave
sintering of the objects. The examples cited above can be extended to other
oxide materials which require similar sintering conditions for densification of
specific components. Further, all the products fired in this system maintain the
desired shape without any warpage and any visible defects. The products have
been characterized by different analytical techniques and found to be in excellent
agreement with the requirement of the products for the particular applications.

WE CLAIM
1. A microwave sintering system for uniform sintering of large volume
ceramic bodies in a microwave field, comprising a microwave heating
apparatus with wave guide; a furnace having a furnace chamber; housing
insulation means including a kiln furniture arrangement for large volume
loading and accommodating the object to be sintered; and for generating
and introducing multiple microwave feed into said furnace chamber,
characterized in that comprising: means for measuring the temperature of
said object contained in said furnace body by a non contact measurement
method, and means for controlling microwave power from multiple
microwave feed source by adapting a programmable controller.
2. A Microwave sintering furnace as claimed in claim 1, comprising a PLC
controlled multifeed microwave source with simultaneous feedback from
the programmable controller which is embedded with safety interlocks.
3. A microwave sintering furnace as claimed in claim 1, wherein said means
for measuring the temperature is fitted with an air purging unit to avoid
moisture condensation during initial heating cycle.
4. A microwave sintering furnace as claimed in claim 1 comprising Online
microwave leak detection system integrated with the control system to
avoid exposure to microwaves during any accidental leakage.
5. A Microwave sintering furnace as claimed in claim 1, further comprising a
stainless steel chamber with ports for microwave feed, exhaust system to
release volatile matter, temperature measurement port; mode stirrers, a
blower for fast cooling; and a cooling water plant, wherein the doors of

the chambers are vertically movable and fitted with limiting switches and
pneumatically controlled.
6. A microwave sintering system as claimed in claim 1, wherein the housing
insulation means comprising layers of vacuum formed insulation boards
embedded with high temperature insulation blankets surrounding the
products to be sintered, and wherein the wall thickness is optimized to
restrict the wall temperature to less than 50 degree C.
7. A microwave sintering system as claimed in claim 6, wherein the high
temperature kiln furniture arrangement is configured in layered structure
with porous susceptor materials disposed in alternate positions to
maintain temperature uniformity during heat treatment at high
temperature.
8. A microwave furnace according to claim 7,comprises of loading of green
products in the kiln furniture assembly in the layered structure , wherein a
central portion of the assembly is provided with a flat product of
composition identical to that of the green product, and wherein the flat
product is used for focusing through infra red pyrometer for temperature
measurement during the heat treatment process.
9. A microwave sintering system as claimed in claim 1, wherein the system is
operable at 2.45 GHZ, capable of achieving a temperature above 1600
degree C, and compatible to the isothermal systems.
10.A microwave sintering method in a system as claimed in claim 1,
comprising:

- heat treatment of alumina-based components in the furnace of the system
at a temperature above 1600 degree C, and
- thermal insulation assembly enclosing the kiln furniture arrangement
containing the products to be isothermally sintered.

The present invention relates to a microwave sintering system for uniform
sintering of large volume ceramic bodies in a microwave field, comprising a
microwave heating apparatus having multiple microwave feed source with wave
guides; a furnace having a furnace chamber; housing insulation including a kiln
furniture arrangement for large volume loading and accommodating the object to
be sintered; measuring the temperature of said object contained in said furnace
body by a non contact measurement method, and means for controlling
generation of microwave power from multiple microwave feed source by
adapting a programmable controller.