FIELD OF THE INVENTION
The present invention relates to a high temperature insulation system for
uniform heat treatment of ceramic components in large volume. The system
comprises of systematic arrangement of kiln furniture and porous microwave
transparent vacuum formed insulation boards in order to maintain isothermal
zone for sintering ceramic components inside a microwave furnace. The design
involves the suitable arrangement and materials for base, wall, door and roof
and to integrate them all as a single structure using microwave compatible
materials capable of withstanding high temperatures in the range of 1600-1700
deg C.
BACKGROUND OF THE INVENTION
Microwave Sintering furnaces have been developed world wide for the heat
treatment of ceramic products at the peak temperatures of 1600-1620 deg C.
The most critical component of the furnace is the high temperature insulation
assembly which plays an important role as the furnace operates at very high
temperatures where heat loss needs to be avoided in order to maintain the
efficiency of the system. Generally, the high temperature furnaces have
insulation cover attached to the furnace wall with metallic anchors which cannot
be used in the microwave furnace. The insulation materials used include certain
specific kiln furniture and alumina fiber insulation boards. The insulation
assembly includes the integration of kiln furniture, loading zone and insulation
boards. For an improved performance of the system, the insulation assembly
should be such that the loading zone has temperature uniformity and the
temperature of outer insulation layer should be less than 50 deg C. The prior art
describe the development and effect of susceptors and isothermal barrier zone
on the sintering of ceramics and temperature uniformity. However, the prior art
is silent on design and configuration of an improved microwave furnace insulator
assembly.
US Patent No 5353567 describes an insulation module assembly and its
installation which are formed by compressing blankets of refractory fibrous
materials into modules. These are attached to the furnace wall by attachment
brackets imbedded internally which are used to secure the module to the shell
using bolts, self tapping screws and studs embedded in bulky solid materials or
welded to the shell in patters to match the insulation module. The prior art
methods have disadvantages of shell penetration, bulkiness, excessive time to
lay out the fastener pattern. This patent provides the design of an insulation
module assembly that can be stud welded directly through the assembly top a
wall that requires insulation using standard electric arc stud welding equipment.
These insulation modules are used in all faces of the furnace to provide a
complete insulation layer along with an inner layer of insulation boards.
US patent 6197243 describes a multi-layered heat distributor system provided for
use in a microwave process. The multi-layered heat distributors includes a first
inner layer of a high thermal conductivity heat distributor material (HTCHD)
material, a second middle layer of insulating material that is chemically
compatible and an optional third insulating outer layer. The multi-layered heat
distributor system is placed around the ceramic composition or article to be
processed and located in a microwave heating system. The HTCHD inner layer
includes a material that has a low absorption of microwaves or is transparent at
the temperature the materials are processed at, has good thermal conductivity,
and is chemically compatible with the article being processed. For example, the
HTCHD material can be formed from BN, Si3N4 , AIN, BeN, SiO3 ,AI2O3 or ZrO2
depending on process conditions.
Japanese patent 240451 discloses a high performance refractory insulating
material suitable as a furnace wall material for firing of calcination materials by
self-heating provided by microwave radiation. A heating layer is provided on one
face of a substrate, the said substrate having an inorganic fiber as the main
constituent, while the heating layer is comprised of Mullite as the chief
constituent. The heating layer may also include, in addition to Mullite, a material
having a microwave absorbing property higher than that of Mullite. Further, the
heating layer includes an inorganic binder. The said inorganic binder is coated as
a continuous film only on Mullite, or on Mullite and the material having a
microwave absorbing property higher than Mullite. The material having a
microwave absorption property higher than that of Mullite regulates the calorific
value of the heating layer. A coating material for constituting the heating layer,
the said coating material characterized by adding one or more types of
substances such as iron oxide, magnesia, zirconia, and silicon carbide, that has a
microwave absorption property higher than that of Mullite. In the baking of a
calcination material by self-heating due to microwave, a much more uniform
baking of the calcination material is possible by surrounding the calcination
material with a refractory insulating material which has microwave absorption
characteristics equivalent to that of the baking material, and controlling the
occurrence of the temperature gradient by radiative cooling of the baking
material.
US Patent 6891140 discloses that when sintering is conducted using microwaves
, a pseudo adiabatic space completely insulating an object to be sintered with a
thermal insulating material which has an equivalent microwave absorption
property to that of the object to be sintered is formed. This restrains the
occurrence of thermal gradient in the object to be sintered due to radiation
cooling and more uniform sintering can be accomplished. The patent further
describes a method of manufacturing sintered products by microwaves which
comprises: i) providing an insulating wall permeable to microwaves, ii) disposing
the objects to be sintered within the chamber by a feeding system and Hi)
forming the sintered objects by radiating microwaves with a microwave
generator via the insulating wall to the objects to be sintered carried in to the
sintering chamber so that the temperature within the sintering chamber is
changed in order to correspond to the sintering process of the objects to be
sintered in the feeding direction of the objects to be sintered.
US patent 233778 -describes a process to prevent the cracking or deformation of
an object to be sintered when degreased by microwave heating. The degreasing
furnace also includes two layers of insulating members outside the space in
which the object to be sintered is positioned, with the innermost member having
a microwave absorption characteristic similar to that of the object to be sintered
because of which the temperature difference between the exterior and interior of
the object can be reduced to achieve more uniform sintering. The outer
insulating member has a smaller specific heat.
US patent 408661 describes a microwave firing furnace comprising of heating
means and a furnace chamber for holding material to be fired containing an
organic binder and further, has a carrier gas introduction pipe for introducing a
carrier gas that contains oxygen at a concentration lower than that of the air to
suppress the burning of the organic binder. The heat insulating member which
forms the furnace chamber and has a poor microwave absorption is formed in a
plurality of layers which are constituted by materials having higher temperature
resistance towards the inside and the innermost layer of the heat insulating
material is having a microwave absorption factor equal to, or larger than that of
the material to be fired.
Motoyasu Sato et al. described the concept of an "isothermal barrier" to inhibit
heat transfer through the surface of the sample in their previous studies of 2.45
GHz microwave sintering of porcelain. The structure of the isothermal barrier
consists of a thin innermost layer with the identical microwave characteristics to
the sample and thick outer layers with low heat conductivity and low microwave
absorption. The isothermal barrier was effective in creating the uniform
temperature distribution in porcelain products. In this study, sintering of alumina
was studied by using 2.45 GHz microwave heating with the isothermal barrier at
various temperatures. The temperature difference within the sample was
predicted from the densities of samples with different sizes, i.e. different volume
to surface ratios. The temperature difference in the sample was estimated from
the difference between the surface (measured) and the interior (calculated).
Though large temperature differences existed at temperatures near 1500 deg C,
the temperature differences became smaller as the temperature reachedl600
deg C.
All the prior art described hereinabove teach the development of an isothermal
barrier zone for the sintering of ceramic components in microwave furnaces. The
use of susceptor for sintering of low dielectric loss ceramics has also been
acknowledged in prior art. But, the prior art is mostly silent regarding design
ofhigh temperature insulation assembly, although a few prior patents discuss the
design of insulation modules and layered structures of insulation assemblies used
in the microwave furnace.
Development of an insulation assembly is very critical in a microwave furnace in
order to avoid the heat loss by insulating materials used for loading furnitures,
since microwave interacts directly with the materials resulting heat in contrast to
indirect heating in the conventional furnaces/kilns. Further, to obtain an uniform
heating zone during heat treatment is crucial for large volume sintering of
ceramic components at high temperature. The materials used in the insulation
assembly have to be selected based on the desired properties. This invention
focuses on designing an independent insulation assembly for microwave furnace
used to sinter green ceramic products at temperatures exceeding 1600 deg C.
OBJECTS OF THE INVENTION
It is therefore, an object of the present invention to propose a high temperature
insulation system for uniform heat treatment of ceramic components in large
volume, in which of a ceramic anchoring means are provided for configuration of
door and wall structures for the insulation assembly.
A further object of the present invention is to propose a high temperature
insulation system for uniform heat treatment of ceramic components in large
volume, in which an uniform heating zone is created with microwave susceptible
materials which are integrated in the insulation wall, door and loading zone
assembly.
Another object of the present invention is to propose a high temperature
insulation system for uniform heat treatment of ceramic components in large
volume, in which loading volume, isothermal zone, base structure, insulation
wall, door and roof assembly are integrated.
SUMMARY OF THE INVENTION
According to the invention, the system comprises a kiln furniture and a plurality
of porous microwave transparent vacuum formed insulation boards forming an
uniform heating zone for sintering ceramic components inside a microwave
furnace at a very high temperature. The system is configured with a base, wall,
door and roof, and assembled as a single structure, the materials for the said
components are capable of withstanding high temperatures in the range of 1600-
1700 deg C.
Therefore, an improved system is provided including ceramic anchor systems for
critical parts like doors and walls, creating a uniform heating zone by composite
materials and integration of different parts of the system.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - Shows the high temperature insulation system according to the
invention.
DETAIL DESCRIPTION OF THE INVENTION
As shown in fig. 1, the system according to the present invention is configured as
a graded insulation assembly structure for high temperature treatment of
ceramic components. The insulation structure constitutes an integration of five
different components, namely, a base structure (1), an insulation wall (2), a door
assembly (3), an insulation roof (4), and a loading zone kiln furniture (6).
Materials suitable for different components (1,2,3,4,5) have been developed and
used. The graded structure consists of a first inner layer of higher thermal
conductivity and microwave absorption characteristics followed by a layer of
primary and secondary microwave transparent materials and an outer insulating
layer which covers the entire structure. The same graded structure has been
used in and around the loading zone on all the sides.
In the present invention, the insulation base structure (1) has been designed
such that the weight of the kiln furniture (5) is reduced to allow better
microwave absorption in an object (7) to be sintered. A composite structure
containing Alumino silicate and alumina materials have been developed which act
as the load bearing columns. This structure further houses a base tile layer which
transfers the weight of the entire assembly to these columns, so that the whole
insulation assembly rests on this base structure (1).
The insulation wall (2) has a graded design which consists of an inner microwave
absorbing layer followed by two layers of primary and secondary microwave
transparent materials with low density, high purity and optimum thermal
conductivity and an outer lower grade insulating layer covering the whole
structure. The uniform heating zone forms a part of the inner layer of this
structure with the help of Recrystallized alumina anchors and high purity ceramic
adhesive.
Moreover, according to the present invention, the insulation door (3) also follows
the graded design with the door not forming a part of the furnace door. The
door structure (3) has been developed in two parts with the hot face lined with
the composite tiles used for creating a uniform heating zone and the outer
surface of the door lined with low dielectric loss ceramic tiles which also provide
a handling arrangement. This Graded door structure consisting of composite heat
absorbing tiles, microwave transparent boards and low dielectric loss tiles is held
together with the help of threaded ceramic anchors(9). The ceramic anchor
consists of a threaded rod and washer designed to take care of the thermal
expansion of the same.
Separate insulation has been provided for the loading zone (5) and the overall
assembly structure in the roof assembly (4). The roof assembly (4) has been
developed in two parts with one part integrated to the inner layer of wall
assembly (2) and the other slotted roof covering the entire structure. The latter
has been positioned with the insulation wall (2) held together by recrystallized
alumina tubes running deep in to the base tile layer of base structure.
The slotted structure of the roof and the positioning of the roof with the base
ensure the stability of insulation assembly with entire weight of insulation roof
being borne by the base structure assembly.
The kiln furniture (6) in the loading zone (5) consists of low dielectric loss
Alumino silicate tiles and high dielectric loss tiles of optimum thickness having
suitable composition of alumina, silica and silicon carbide. Higher dielectric loss
materials are used as the covering structures for creation of uniform heating
zone and lower dielectric loss materials are used as the holding structures in the
kiln furniture assembly. The loading zone structure (5) is in contact with the
uniform heating zone (8) formed in the insulation wall, (2) and door assembly
(3), base structure and the roof assembly. Proper arrangements have been
provided in the uniform heating zone tiles to use recrystallized ceramic pins and
threaded ceramic anchors for holding the graded structures together in different
parts of the insulation assembly. The loading zone (5) has an independent roof
consisting of low dielectric loss ceramic tile which makes the loading zone
assembly and disassembly convenient. Arrangements have been made in the kiln
furniture tiles to measure the temperature of the sample positioned in any of the
loading layers. Thin tiles having Alumina based compositions are being used for
temperature measurement with emissivity values in the range of 0.35-0.45.
EXAMPLES:
The present invention is further explained concretely with working example;
however, this is only for illustration and does not limit this invention.
EXAMPLE 1
45-50 Kg's of Alumina based grinding media balls fabricated by isostatic pressing
with green diameters in the range of 30-50mm were loaded in the kiln furniture
assembly. This assembly is enclosed on all sides by the uniform heating zone.The
heating time was 9-10h to reach a peak temperature in the range of 1590-1610
deg C followed by a 2 hour soaking at that temperature. Infra red pyrometer
was used to measure the temperature. The products after sintering yielded bulk
density in the range of 3.85-3.89g/cc with <0.1 % water absorption. The
products were tested for Vicker's hardness and the corresponding values were
found to be superior to that of conventionally processed samples. The density of
the product in each layer was found to be similar indicating the uniformity of
temperature throughout the entire volume.
Example 2
In another experiment, 45-50 Kg's of alumina tubes 150 mm long and 20 mm
OD and 5 mm ID were loaded in a three layer loading pattern for sintering in the
Microwave furnace. Proper care was taken to avoid any contamination of the
alumina samples. The products were heated similarly to the process described
in example 1 at a peak temperature in the range of 1590-1610 deg C followed by
a two hour soaking. At peak temperature, the applicator wall temperature was
measured and found to be below 50 deg C.The furnace door was opened and
the temperature of the outermost insulation layer was measure to be around 50
deg C indicating the effectiveness of the insulation assembly inside the cavity.
Moreover, the cooling rate of the fired materials as indicated by the
programmable controller was very low indicating the improved heat retention
capacity of the iso thermal zone materials at peak temperature. The products
after firing resulted density in the range of 3.85-3.93g/cc with water absorption
les than 0.1%.
Example 3
In another experiment, 45-50 Kg's of alumina products consisting of grinding
media balls and tubes were loaded in the kiln furniture assembly. The products
were heated similarly to the process described in above stated examples but the
peak temperature was increased in the range of 1630-1640 deg C followed by a
soaking of 15-30minutes. The fired products resulted density in the range of
3.80-3.90g/cc with water absorption less than 0.1%.
Similar experiments were repeated with green alumina products in batches of
45-50 Kg's.Sintering of similar products was carried out with increased soaking
times of 3 and 4 hours at peak temperature in the range of 1590-1610deg C, but
there was no considerable increase in the density values. A consistency in
density and water absorption values has been observed with no visible defects,
as such. The examples cited above can be extended to other oxide materials
which require similar sintering conditions for densification of specific
components.
WE CLAIM
1. A high temperature insulation system for uniform heat treatment of
ceramic components in large volume, comprising a base structure, an
insulation wall, a uniform heating zone, an insulation door, a roof
structure, a plurality of threaded ceramic anchors, and a kiln furniture for
a loading zone assembly; wherein the insulation wall is , a graded
insulation wall consisting of two intermediate layers and an outer layer,
the intermediate layers being formed by transparent materials with low
density, high purity with optimal thermal conductivity, the outer layer
constituting a lower grade insulating layer, and wherein the insulation
door having a graded structure and formed in two parts and held by
threaded ceramic anchors, and accommodating high dielectric loss tiles on
the hot face and low dielectric loss on the outer face.
2. The insulation system as claimed in claim 1, wherein the base structure
constitutes a composite structure formed of alumino silicate and alumina
materials acting as the load bearing columns and accommodating tile
layers, the configuration of the base structure enabling reduction in
weight of the kiln furniture to allow improved microwave absorption of the
object to be treated;
3. The insulation system as claimed in claim 1, wherein the roof structure is
configured with two parts, a first part integrated to the inner layer of the
insulation wall, the second part with slotted configuration covering the
entire system structure with recrystallized alumina tubes used for
positioning the base structure ,insulation wall and insulation roof together.
4. The insulation system as claimed in claim 1, wherein the kiln furniture is
disposed in the loading zone consisting of tiles with higher dielectric loss
tiles having suitable composition of alumina, silica and silicon carbide as
the covering structures for creation of uniform heating zone and lower
dielectric loss materials as the holding structure.
5. The insulation system as claimed in claim 1, wherein the uniform heating
zone forms a part of the inner microwave absorbing layer in the insulation
wall, door, base and roof, supported by a plurality of recrystallized
alumina pins, threaded ceramic anchors and high purity ceramic adhesive,
and wherein the uniform heating zone comprises of high dielectric loss
tiles of optimum thickness having suitable composition of alumina, silica
and silicon carbide.
6. The insulating system as claimed in claim 1, wherein the loading zone
structure is in contact with the uniform heating zone, and comprises low
dielectric loss ceramic tiles to configure an independent roof other than
the roof consisting of vacuum formed microwave transparent materials.

The invention relates to a high temperature insulation system for uniform heat
treatment of ceramic components in large volume, comprising a base structure,
an insulation wall, a uniform heating zone, an insulation door, a roof structure, a
plurality of threaded ceramic anchors, and a kiln furniture for a loading zone
assembly; wherein the insulation wall is , a graded insulation wall consisting of
two intermediate layers and an outer layer, the intermediate layers being formed
by transparent materials with low density, high purity with optimal thermal
conductivity, the outer layer constituting a lower grade insulating layer, and
wherein the insulation door having a graded structure and formed in two parts
and held by threaded ceramic anchors, and accommodating high dielectric loss
tiles on the hot face and low dielectric loss on the outer face.