Electric heating – Microwave heating – With diverse-type heating
Reexamination Certificate
1996-05-22
2001-01-09
Leung, Philip H. (Department: 3742)
Electric heating
Microwave heating
With diverse-type heating
C219S685000, C219S700000, C219S715000, C219S718000
Reexamination Certificate
active
06172346
ABSTRACT:
INTRODUCTION
The present invention relates to the microwave-assisted processing of ceramic materials. To that end there is described a microwave furnace and a method of operating a microwave source, an enclosure for the confinement of the microwave and for containing an object to be heated, means for coupling the microwave source to the enclosure and independently controllable alternate heating means disposed in relation to the enclosure to provide at least one of radiant and convective heating within the enclosure.
The volumetric heating of ceramic materials using microwaves can in principle be used to overcome many of the difficulties associated with the inherently poor heat transfer characteristics of ceramic components and certain glasses such as dark colored glass or glass ceramics. The following will therefore primarily concentrate on ceramic materials and components, however it will be appreciated that the teaching provided is also applicable to any material which requires heating and exhibits a low thermal conductivity, or because the rate determining factor in the heating of a material will be one of mass transfer, a low diffusivity.
Problems Associated With the Conventional Firing of Ceramics and Glass
In the sintering of ceramics, high temperatures are necessary to overcome the activation energy barrier for the various mass transfer processes involved in the reduction in the particle surface area. In general the rate of mass transfer is dependant on the sintering temperature, with higher temperatures giving rise to more rapid densification. In glass and glass-ceramics the main limitation is also one of mass transfer, as the infra-red radiation cannot significantly penetrate the surface of the glass, particularly if the glass is colored.
In a conventional furnace, heat transfer is predominantly by radiation to the surface of the component followed by conduction from the surface to the center. During sintering the high radiant loading required to accomplish the task in a fixed time period results in the temperature gradient across the specimen, which is determined primarily by the thermal conductivity, becoming steep with the surface at a much higher temperature than the center. Since the thermal conductivity of a typical unsintered homogeneous material is extremely low (i.e. less than 1W/mK), there is a tendency in larger components for the temperature gradient to result in a large thermal mismatch between the centre and the surface. This in turn leads to the development of a stress at the surface of the component which is proportional both to E
eff
, the effective Young's Modulus, and the thermal expansion co-efficient, a which for ceramics is typically of the order of 8×10
-6
K
-1
.
During the first stages of heating, if the component is heated uniformly on all sides, the described thermal mismatch generates compressive thermal stresses at the surface, and as a result the propagation of cracks is inhibited and failures are comparatively rare. However if the thermal gradient is sufficiently large, cracking will occur. Unfortunately it has been found that this maximum temperature gradient varies with both the material and temperature making it difficult to predict and necessitating extensive trials to determine the optimum firing schedule for each component and material composition.
Where the radiant heating is non-uniform, for example, if the components are stacked or the subject of multiple firing, one side of the component will be in tension and prone to crack propagation. In extreme cases catastrophic fracture can occur while in less severe cases the stresses may still cause distortion of the components.
Whether the heating is either uniform or non-uniform, once the sintering regime is reached, the linear shrinkage associated with the densification process tends to over shadow the thermal mismatch and allow some relaxation of the thermal stresses with the consequence that the stress distribution becomes more complicated to predict.
The problem of poor heat transfer is accentuated still further with fast firing. In many cases it is preferable to ‘fast fire’ a ceramic in order to promote better energy efficiency and allow greater throughput of components. At the same time by fast firing it is also possible to achieve an improved fine-grained microstructure as it has been found that slower heating rates tend to give rise to a coarser grain size and a deterioration in mechanical strength since the larger grains act as flaws. However, in order to rapidly fire a component in a short time period it is necessary to use high radiant surface loadings and this in turn provides the heating elements with a difficult role to perform and shortens their life expectancy considerably necessitating the use of higher rated and more expensive elements. At the same time it also promotes temperature gradients within the components as the heat conduction is typically slow.
The presence of a severe temperature gradient, apart from causing cracking, can also result in uneven sintering, with the surface sintering before, and at a faster rate than the centre. This can result in non-uniform properties within the material, which can make a predetermined quality specification difficult to meet and can lead to the generation of a lot of waste material, particularly if the final component is to be machined from a larger block. It can also prevent the escape of binders and other volatile species.
Furthermore, in the firing of ceramics, it is not uncommon to encounter major crystallographic phase transformations which can also be accompanied by volume changes. For example, in the firing of quartz-containing clay bodies for use as tableware, the inversion of &agr;/&bgr; quartz requires a uniform temperature profile throughout the component. This requires specific firing schedules which for the above reasons are clearly difficult to attain and so restricts the design of the components.
In summary therefore the theoretical limits of conventional heating techniques impose relatively modest heating rates on components having anything other than a small cross-section. Consequently in some parts of the ceramic industry, firing schedules lasting over 2 weeks are used for large components and, as a result, extremely large tunnel-style kiln systems have to be used in order to achieve the required throughput. This makes the firing process not only energy intensive, which naturally tends to favor non-electrical methods of heating, but also highly capital and labor intensive. These factors all contribute to slow the uptake of new processes and products which involve the sintering of ceramics since at the same time these processes and products significantly lower the net value to the company of the project concerned.
Problems Associated With the Microwave-Only Firing of Ceramics and Glass
In the past microwave processing has been suggested as an alternative to conventional radiant furnaces in an attempt to address the problems outlined above. The principal benefit offered by microwave processing is that by depositing energy directly within the component it is possible to overcome the problems of heat transfer within individual components and throughout the furnace.
The benefits of so called volumetric heating techniques have long been recognised in other industries. For example, it is known to use RF in drying processes and to use lower frequencies in the heat treatment and melting of metals. However ceramics, being principally non-conductive, require the use of the much higher frequencies of the microwave band before effective coupling can be achieved. Attempts have been made to heat ceramics at microwave frequencies of between 900 MHz and 30 GHz and it has been found that many materials will couple quite efficiently at these frequencies. Consequently microwave heating has been cited as a way in which temperature uniformity can be improved within a material.
In theory, by depositing the energy directly within the component the problems of heat transfer throughout individual components and throughou
EA Technology Limited
Leung Philip H.
Westman Champlin & Kelly P.A.
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