WO1993016571A1

WO1993016571A1 - Microwave processing materials

Info

Publication number: WO1993016571A1
Application number: PCT/GB1993/000238
Authority: WO
Inventors: John Robert Brandon
Original assignee: Electricity Association Technology Limited
Priority date: 1992-02-07
Filing date: 1993-02-05
Publication date: 1993-08-19
Also published as: GB9202653D0; GB2263967A; AU3458993A

Abstract

A microwave furnace (10) is provided comprising an enclosure (12) for the confinement of microwaves, means defining an aperture (20) in a wall (18) of the enclosure (12), and at least one radiant heating element (16) moveable between a first position in which the heating element (16) extends into the enclosure (12) through the aperture (20) and a second position in which the heating element (16) is withdrawn from the enclosure (12). There is also provided a radiant heating element (100) for use within a microwave furnace (110) comprising a first material (106) that is substantially susceptable to microwaves at ambient temperatures and a second material (102) having a greater thermal conductivity than the first material (106) in order to distribute the heat that in use is generated within the first material (106). There is also provided a microwave furnace (10) comprising an enclosure (12) for the confinement of microwaves, means defining a hot zone within said enclosure (12) and a plurality of radiant heating elements (16) tthat extend through and intersperse the hot zone. There is also provided a batch processing system (700) comprising a plurality of microwave enclosures (702, 704, 706), means (708) for providing each of the enclosures (702, 704, 706) with low level heating, a high power microwave source (712) and means (718) for coupling the high power microwave source (712) to any selected one of the enclosures (702, 704, 706).

Description

MICROWAVE PROCESSING MATERIALS

The present invention relates to the microwave processing of materials and in particular, but not exclusively, to the microwave heating of ceramics.

In recent years the microwave processing of materials has aroused widespread interest as an alternative to the heating of materials using conventional techniques such as resistance heating or the burning of fossil fuels. In comparison with these techniques microwave heating is a fundamentally different process in which heat is instantaneously generated within a material rather than originating from an external source.

Microwaves form part of the electromagnetic spectrum and comprise those waves having a frequency within the range from 0.3 to 300 GHz and a corresponding wavelength in the range from l to lmm. As may be expected therefore, microwaves obey the laws of optics and may be transmitted, absorbed or reflected depending on the material on which they are incident. However, unlike most electromagnetic radiation within the visible region, microwaves are coherent and polarized.

When microwaves penetrate and propagate through a dielectric material, the internal electric fields generated within the affected volume induce translational motion in charges such as electrons or ions and rotate charge complexes such as dipoles. The resistance to these induced movements due to inertial, elastic and frictional forces results in the attenuation of the electric fields and causes the volumetric heating of the material. As a result of this mechanism, the thermal gradients and flow of heat in microwave-processed materials are the reverse of those in materials processed by conventional heating methods. Consequently, microwave processing makes it possible to heat bodies both rapidly and uniformly with the potential advantages to industry of being able to significantly reduce manufacturing costs as a result of shorter processing times, of being able to improve product uniformity and yields, and of being able to produce improved or unique icrostructures with enhanced properties.

These advantages have not been lost on those working in the field of ceramics where microwave processing has been used in process control and non-destructive evaluation (NDE) to detect moisture, defects and pores; in plasma processing in both microwave-plasma chemical vapour deposition (MPCVD) and microwave-plasma-induced sintering (MPIS) ; in liquid state processing to process, synthesise and analyse materials in solutions and suspensions; in low temperature solid state processing to remove both water and organic binders; and in high temperature solid state processing to fire, sinter and melt.

The use of microwaves is not however without its problems. Not all materials are equally susceptible to microwaves and therefore processing variables such as the power level, the heating time and the efficiency of the energy transfer are material dependant. In composite materials this can lead to selective heating which in turn may produce stresses within the composite and ultimately cracking. Furthermore the ability of a material to absorb microwave energy is dependant on the temperature of the material concerned. Above a critical temperature this can lead to a process known as thermal runaway in which hot spots in the material absorb microwave energy more efficiently than their cooler neighbours making the hot spots hotter and consequently able to absorb microwave energy still more efficiently. The net result is an exponential increase in temperature which, if limited to certain regions of the material, can lead to considerable internal stresses and often to the complete destruction of the heated material.

A related problem is that not all materials are able to efficiently absorb microwave energy at ambient temperatures but must first be heated to a more elevated temperature. The raising of the material to this more elevated temperature using microwave heating can be a time consuming and inefficient process since to start off with it is characterised by a long dwell period in which only a small rise in temperature is evident.

These factors, coupled with the fact that microwave heating is more expensive than conventional techniques, have, on the whole, limited the application of microwave processing to small scale, laboratory use. The present invention is directed to the overcoming of the foregoing problems and to the provision of a microwave heating apparatus that may be used on an industrial scale.

In the past, and on a laboratory scale, attempts have been made to overcome the problems associated with the poor suceptibility to microwaves of many materials at ambient temperatures by heating the materials concerned with a combination of microwave and resistive heating techniques. By first heating a material using conventional techniques until it reaches a temperature at which microwave heating may take over a far more efficient overall method of heating is achieved. Because it is widly recognised that at high temperatures the presence of resistive heating elements within a microwave enclosure would lead to unacceptable problems with microwave leakage and with arcing, both between the elements themselves and also between the elements and the walls of the enclosure, most attention has been directed to the provision of a microwave enclosure having conventional external heating elements. The resulting enclosure is however prone to a number of further problems. Firstly, the enclosure is significantly more expensive than those that are not provided with external heating means as a result of the additional material and manufacturing costs; secondly, it exhibits a slower response to rapid heating rates because of the increased thermal resistance of the cavity; and thirdly, the maximum temperature that may be achieved is limited by the materials of which the enclosure is made.

According to a first aspect of the present invention there is provided a microwave furnace comprising an enclosure for the confinement of microwaves, means defining an aperture in a wall of the enclosure, and at least one radiant heating element oveable between a first position in which the heating element extends into the enclosure through the aperture and a second position in which the heating element is withdrawn from the enclosure. Advantageously the said at least one radiant heating element comprises a resistive heating element and in particular may comprise two coaxial quartz tubes with a length of resistive heating wire disposed therebetween, the resistive heating wire preferably comprising a kanthal type heating wire. Alternatively, the said at least one radiant heating element may comprise a microwave susceptor.

Advntageously a guide is provided to facilitate the movement of said at least one radiant heating element between said first and second positions.

Advantageously the aperture has a diameter of less than 0.7 of the wavelength of the microwaves to be confined in the enclosure.

Advantageously the said at least one radiant heating element is held by means of a transverse member capable of sliding relative to one or more supporting pillars that extend substantially perpendicularly to said wall of the enclosure.

Advantageously there is further provided a temperature sensor moveable between a first position in which the sensor extends in to the enclosure through an aperture provided in a wall thereof and a second position in which the sensor is withdrawn from the enclosure.

The use of materials that are susceptible to microwaves at ambient temperatures in order to heat materials having a lower susceptibility is not in itself new. In the past, for example, a material to be heated has often been doped with a more susceptible material in order to increase the ability of the composite to absorb microwaves. While this technique enables the composite material to be heated purely by microwave heating and negates the use of resistive heating elements, the microwave absorption rate may still be limited at low temperatures. In addition the presence of the dopant material may adversly affect the properties of the composite and lead to difficulties with contamination.

On a laboratory scale these problems have in part been addressed by the provision of a microwave susceptor that completely surrounds the material to be heated. At low temperatures the susceptor absorbs microwaves in preference to the sample material with the result that the susceptor is efficiently heated. This heat is then transferred to the sample material by a process of radiation thereby providing the furnace with the benefits previously described in relation to resistive heating elements but without the expense of having to equip the furnace with conventional heating means.

The adaption of this technology for use on an industrial scale is however a complex task and one that requires the solving of some as yet unaddressed problems. For example, on a laboratory scale it is possible to provide a susceptor that completely surrounds the material to be heated but this is clearly not practical when heating large bodies or large numbers. How then is it possible to ensure the uniform heating of the body or bodies concerned and so avoid the generation of thermal stresses?

According to a second aspect of the present invention there is provided a radiant heating element for use within a microwave furnace comprising a first material that is substantially susceptible to microwaves at ambient temperatures and a second material having a greater thermal conductivity than the first material in order to distribute the heat that in use is generated within the first materic .

Advantageously said first material is contained within a substantially cylindrical tube formed from said second material. This tube may be adapted to be mounted by one end from a wall of the microwave furnace and closed adjacent said one end by a refractory plug. Alternatively, the tube may be free standing within the microwave furnace and closed at opposite ends by respective refractory plugs. In either case the or each refractory plug may be formed of refractory alumina, zirconia cement or ceramic fibre.

In another embodiment said first material may comprise a plurality of beads coaxially mounted on a rod formed from said second material. In such an embodiment the pairs of adjacent beads are preferably held in mutually spaced relationship by means of a spacer formed from said second material.

In any of the foregoing embodiments a surface of said second material may be provided with a silicon carbide coating, the silicon carbide coating preferably comprising a plurality of spaced apart coated regions.

In any of the foregoing embodiments said first material may comprise zirconia or a zirconia mixture, powdered graphite or silicon carbide. In any of the foregoing embodiments said second material may comprise alumina.

According to a third aspect of the present invention there is provided a microwave furnace comprising an enclosure for the confinement of microwaves, means defining a hot zone within said enclosure and a plurality of radiant heating elements that extend through and intersperse the hot zone.

If microwave heating is to be applicable on an industrial scale not only do problems have to be addressed relating to the microwave characteristics of the items to be heated but attention must also be directed to the sheer numbers involved.

Conventional furnaces can be divided into one of two categories; continuous or tunnel furnaces in which the components to be heated pass at a fixed rate through a furnace having a number of independent zones in order to produce the desired thermal profile, and batch furnaces in which the components are static and the conditions of the furnace are varied. Of these two categories, the former permits the processing of relatively large numbers of components at a fixed load factor but suffers from the disadvantage of inflexible processing conditions during continual use while the latter provides flexible processing conditions but at the cost of low throughputs relative to their peak power consumption and low overall load factors. Some continuous furnaces, particularly those of older design, also suffer ^• from the disadvantage of possessing a transport system that has a high thermal mass. This can lead to poor thermal efficiency and cross contamination if more than one material is to be processed. Low thermal mass batch furnaces can have high thermal efficiencies and can be used to avoid the problems of cross contamination although the employment of furnaces dedicated to a specific task can result in inefficient utilisation, particularly if production demands are continuously fluctuating.

The implications of this are to cast grave financial doubts on the feasability of employing microwave heating on an industrial scale despite the technical benefits that have been outlined above. Microwave heating is able to provide much faster heating rates but this is not appropriate to every stage of a typical furnace heating profile while the cost of replacing each of a group of dedicated batch furnaces with a microwave furnace soon becomes prohibitive.

According to a fourth aspect of the present invention there is provided a batch processing system comprising a plurality of microwave enclosures, means for providing each of the enclosures with low level heating, a high power microwave source and means for coupling the high power microwave source to any selected one of the enclosures.

Advantageously the means for providing each of the enclosures with low level heating may comprise a plurality of dedicated low power heaters. Alternatively, the means for providing each of the enclosures with low level heating may comprise a low level heater and means for coupling the low level heater to any selected one of the enclosures. In either event, the low power heater may comprise a low power microwave source or a radiant heater, the radiant heater in turn comprising either a microwave susceptor or a retractable resistive heating element. A number of embodiments of the various aspects of the present invention will now be described by way of example with reference to the accompanying drawings in which:

Figure 1 is a schematic view of a microwave furnace in accordance with the first aspect of the present invention;

Figure 2 is a cross-sectional view of a radiant heating element in accordance with a first embodiment of the second aspect of the present invention;

Figure 3 is a cross-sectional view of a radiant heating element in accordance with a second embodiment of the second aspect of the present invention;

Figure 4 is a cross-sectional view of a radiant heating element in accordance with a third embodiment of the second aspect of the present invention;

Figure 5 is a cross-sectional view of a radiant heating element in accordance with a fourth embodiment of the second aspect of the present invention;

Figure 6 is a perspective view of a radiant heating element in accordance with a fifth embodiment of the second aspect of the present invention;

Figure 7 is a cross-sectional view of a radiant heating element in accordance with a sixth embodiment of the second aspect of the present invention; Figure 8 is a temperature-time graph illustrating a typical three stage furnace profile used in the firing of ceramics;

Figure 9 is a schematic view of a batch processing system in accordance with a first embodiment of the fourth aspect of the present invention;

Figure 10 is a schematic cross-sectional view of a furnace used in the batch processing system of Figure 9;

Figure 11 is a schematic view of a high power microwave source used in the batch processing system of Figure 9; and

Figure 12 is a schematic view of a batch processing system in accordance with a second embodiment of the fourth aspect of the present invention.

Referring initially to Figure l there is shown a microwave furnace 10 comprising an enclosure 12 for the confinement of microwaves and a framework 14 that provides support to one or more radiant heating elements 16.

The enclosure 12 may be of any conventional construction into which microwaves are introduced from a source (not shown! by means of an appropriate coupling such as a waveguide or coaxial cable (also not shown) . However unlike conventional microwave enclosures, an upper wall 18 of the enclosure 12 is provided with a number of apertures 20 capable of receiving the heating elements 16 while at the same time having a diameter of less than 0.7 of the wavelength of the microwaves to be confined in the enclosure 12 so as to not give rise to problems with radiation leakage. Each of the apertures 20 function as a choke in that they attenuate the microwave field along their length. The properties of such chokes are well known and as a general rule the narrower the aperture the shorter need be its length in order to attenuate the field sufficiently. In the light of this the apertures 20 preferably have a diameter of approximately 12cm when 2450MHz microwaves are used and approximately 30cm when 915MHz microwaves are used.

As can be seen from Figure 1, the framework 14 comprises a base plate 22 fixed to the upper wall 18 of the enclosure 12 and two supporting pillars 24 and

26 disposed at opposite ends of the base plate 22 and extending substantially perpendicularly thereto in a direction away from the enclosure 12. Like the upper wall 18, the base plate 22 is provided with a number of apertures 28 each of which communicates with a respective one of the apertures 20. A similar number of non-conductive and heat-resistant quartz bosses 30, each having a central bore 32, are also provided and in such a way that the central bore 32 of each boss 30 communicates with apertures 20 and 28.

A transverse member 34 disposed substantially parallel to the base plate 22 is loosely supported between the two supporting pillars 24 and 26 and is prevented from sliding therealong under the influence of gravity by the action of respective sliding bushes

36 and 38. A number of apertures 40 are provided in the transverse member 34 each of which is aligned with the central bore 32 of a respective one of the σuartz bosses 30. The heating elements 16 comprise two substantially cylindrical coaxial quartz tubes (not shown) the inner of which acts as a former for the winding of a kanthal type heating wire (also not shown) . Each is received within a respective one of the apertures 40 provided in the transverse member 34, and held in place by means of a clamp 42. At an upper end 44 the heating elements 16 are connected to an electrical power supply (not shown) while at a lower end 46 each is received within the central bore 32 of one of the quartz bosses 30.

A further series of aligned apertures are provided in the upper wall 18, the base plate 22 and the transverse member 34 in order to receive a narrow quartz tube 48 housing a temperature sensor (not shown) connected to suitable monitoring equipment (also not shown) .

In use, in order to heat a material placed within the enclosure 12, the bushes 36 and 38 are slid along their respective supporting pillars 24 and 26 and caused to abut the base plate 22. The transverse member 34 is then lowered until it abuts an upper surface of each of the sliding bushes 36 and 38 and/or an upper surface of each of the quartz bosses 30. In this way the lower end 46 of each of the heating elements 16 is progressively received within apertures 28 and 20 before then extending into the enclosure 12. The same process of lowering the transverse member 34 also serves to introduce into the enclosure 12 the narrow quartz tube 48 that houses the temperature sensor (not shown) .

The electrical power supply to which the heating elements 16 are connected is then actuated causing the elements 16 to heat up as a consequence of the electrical resistance exhibited by the kanthal type heating wire. This heat is radiated within the enclosure 12 and the resulting rise in temperature monitored by means of the temperature sensor contained in the narrow quartz tube 48.

When the temperature of the material to be heated has been elevated to such an extent that the material becomes sufficiently susceptible to microwaves for microwave heating to represent the most efficient method of further raising its temperature, the heating elements 16 are retracted from the enclosure 12. This is achieved by raising the transverse member 34 and sliding bushes 36 and 38 away from their engagement with the base plate 22. Once the lower end 46 of each of the heating elements 16 is withdrawn from the enclosure 12 and is received within the central bore 32 of one of the quartz bosses 30, microwaves are introduced into the enclosure 12 from the source (not shown) where they are absorbed by the material to be heated.

From the foregoing it will be apparent to those skilled in the art that the described embodiment possess many of the advantages of microwave enclosures having conventional external heating elements without also possessing the disadvantages with which such enclosures are associated. Indeed, in direct contrast to the enclosures of the prior art, the described embodiment not only provides a rapid response to increased rates of heating but is also not limited in terms of its maximum operating temperature by the materials of which the enclosure 12 is made.

It will also be apparent to those skilled in the art that the design of the heating elements 16 is such as to be simple and inexpensive to construct whilst also being of sufficiently small cross-section to permit access to the enclosure 12 without giving rise to problems with radiation leakage. Furthermore the quartz tubes and kanthal winding provide the heating elements 16 with good thermal shock properties enabling them to be removed from the enclosure 12 at high temperatures without suffering any ill effects. The quartz bosses 30 facilitate the insertion of the heating elements 16 into the enclosure 12 whilst their non-conductive nature prevents the lower ends 46 of the heating elements 16 from acting as microwave antennae when the heating elements 16 are retracted from the enclosure 12 thereby keeping any leakage of microwave energy to a minimu .

It will also be apparent to those skilled in the art that the resistive heating elements described could be replaced by microwave susceptors capable of efficiently absorbing microwave radiation at ambient temperatures.

One such susceptor 100 is shown in Figure 2 to comprise a substantially cylindrical alumina tube 102 closed at one end 104 and containing a quantity 106 of zirconia or a mixture of zirconia and certain metal oxides such as magnesia, yttria or calcia, the zirconia or zirconia mixture 106 being present in a fibrous, powdered or granular form. The susceptor 100 projects into the enclosure 108 of a microwave furnace 110 by passing through an aperture 112 provided in a wall 114 of the enclosure 108 and then through two further aligned apertures 116 and 118 provided in respective first and second insulating layers 120 and 122. The susceptor 100 is held in place by means of a clamp 124 and is provided at a proximal end with a refractory plug 126 of refractory alumina, zirconia cement or ceramic fibre. In use the alumina tube 102 provides the susceptor 100 with a degree of mechanical strength and on one level acts simply as a support to the loosley packed zirconia or zirconia mixture 106. Being substantially transparent to microwaves, the alumina tube 102 does not contribute to the attennation of the flux of microwaves which, at low temperatures at least, are predominantly absorbed by the zirconia or zirconia mixture 106. However as the zirconia or zirconia mixture 106 is heated, heat is transferred to the alumina tube 102 which then radiates it, thereby heating the sample material placed within the furnace 110.

Because the alumina tube 102 is a better thermal conductor than the zirconia or zirconia mixture 106, the alumina tube 102 is able to act on a second level as a moderator, conducting away any excess heat generated by hot spots in the zirconia or zirconia mixture 106 which have a tendancy to form spontaneously. By redistributing the excess heat in this way it is possible for the susceptor 100 to provide a more uniform heating effect whilst minimising risk of cracking due to thermal stress.

In use the refractory plug 126 serves to reduce heat loss from the zirconia or zirconia mixture 106 and prevent it from fusing with either of the first or second insulating layers 120 or 122 that line the interior of the enclosure 108 and which are typically fabricated of fibreboard. The fact that the alumina tube 102 is closed at 104 and separated from an opposite wall (not shown) of the enclosure 108 by an air gap similarly prevents the zirconia or zirconia mixture 106 from coming into contact with and melting the said opposite wall. In a different embodiment shown in Figure 3 the susceptor 200 no longer projects into the microwave enclosure 202 by passing through an aperture but instead is adapted to stand unsupported on a floor of the enclosure 204. As with the previous embodiment however, the susceptor 200 comprises a substantially cylindrical alumina tube 206 filled with zirconia or zirconia mixture 208 in a fibrous, powdered or granular form. The alumina tube 206 is closed above and below the zirconia or zirconia mixture 208 by respective first and second refractory plugs 210 and 212 which, as before, may be of refractory alumina, zirconia cement or ceramic fibre. The first of the two plugs 210 is disposed at an upper end 214 of the tube 206 while the second plug 212 is spaced a short distance from a lower end 216. In this way a small air gap 218 is provided between the second plug 212 and the floor 204 thereby ensuring that the hot zirconia or zirconia mixture 208 does not come into contact with the walls of the enclosure 202. The height of the air gap 218 is preferably greater than or equal to the internal diameter of the alumina tube 206 and in the embodiment shown has a dimension of approximately 20mm.

In use the alumina tube 206 and the zirconia or zirconia mixture J208 both behave in the same way as was described with reference to the embodiment shown in Figure 2. However both that embodiment and the embodiment shown in Figure 3 suffer from the disadvantage of having a relatively poor thermal shock resistance. What is more the high thermal mass of the alumina tubes 102 and 206 restricts the use of the susceptors 100 and 200 to applications involving slow heating and cooling rates in order to prevent fracture. Having said that however, the embodiment shown in Figure 3 is in compression along its axis and as a result is more resistant to catastrophic failure caused by thermal stress cracks. The cracks sometimes form but they are not critical to the operation of the susceptor. On the other hand both embodiments do possess the advantage that, when in use, the alumina tubes 102 and 206 are not in contact with the hottest part of the zirconia or zircnoia mixture 106 or 208; the centre. By surrounding but not actually passing through a central region of the zirconia or zirconia mixture 106 or 208, the alumina tubes 102 and 206 are able to withstand higher surface loadings and as a result the susceptors 100 and 200 can operate at increased temperatures of up to 1900°C.

In a further embodiment shown in Figure 4 the susceptor 300 comprises an alumina rod 302 received within a recess 304 provided in a surface 306 of a microwave enclosure 308 and upon which are mounted a plurality of beads 310 of zirconia or zirconia mixture, each having a central bore 312 and being separated from adjacent beads 310 by an alumina spacer 314. A somewhat larger alumina spacer 316 is provided at a lower end of the alumina rod 302 to prevent the bead 310 closest to the surface 306 from fusing with the lining of the enclosure 308.

In use the alumina rod 302 and the beads 310 of zirconia or zirconia mixture behave in a manner analogous to the alumina tubes 102 and 206 and zirconia or zirconia mixture 106 and 208 of the two preceding embodiments. Nevertheless in comparison with the susceptors of Figures 2 and 3 the susceptor 300 shown in Figure 4 possess not only a larger surface area to volume ratio but also an improved resistance to thermal shock. It is acknowledged however that the relative geometry of the rod 302 and the beads 310 is critical to the establishment of both these advantages. Thus on the one hand the beads 310 must be sufficiently small to keep the build-up of internal heat to a minimum and thereby prevent the generation of large thermal stresses while on the other hand the central bore 312 must not be so small as to cause the heat built up inside the bead 310 to melt the alumina rod 302. Likewise if the alumina rod 302 is not of a large enough diameter it will have a tendancy to distort at high temperatures and will become prone to melting as a result of its inability to conduct heat away from the beads 310 sufficiently quickly. This last condition is partially eased by the use of alumina spacers 314 between the beads 310 which not only prevents the beads 310 from fusing together but also facilitates the redistribution of heat and aids its subsequent radiation. Even so tests have shown that while a zirconia density of approximately 1.5g/cm² offers a good balance between susceptibility and thermal stress resistance, the beads 310 need to be limited to a diameter of less that 20mm in cases where zirconia fibre is used.

A further design of susceptor 400 is shown in Figure 5 to comprise a generally cylindrical rod 402 mounted on a wall 404 of an enclosure 406 by means of a refractory support 408. The cylindrical rod 402 may be of fully dense zirconia in which case the susceptor 400 may display a limited thermal shock resistance or alternatively may be doped with a quantity of graphite or silicon carbide powder in order to improve its low temperature susceptibility and enhance the uniformity of heating. An addition of 20wt% of graphite or silicon carbide power has been found to be ideal for many applications although some compromise in high temperature capability is sometimes necessary to prevent the oxidation of the graphite or silicon carbide.

In a different embodiment the cylindrical rod 402 may be formed of porous silicon carbide which has been found to be susceptible to microwaves at a wide range of temperatures and have a sufficient resistance to thermal shock to prevent fracture.

Silicon carbide is used in the form of a paste in yet a further embodiment shown in Figure 6. As can be seen the susceptor 500 comprises a substrate 502 having a surface 504 to which is applied a plurality of spaced apart silicon carbide coated regions 506, each separated from its neighbours by a narrow gap 508.

In use the silicon carbide coated regions 506 are heated by the absorption of microwaves and this heat is then radiated by the susceptor 500. The low thermal mass of the coated regions 506, their rapid response and their ease of fabrication make the susceptor 500 suitable for a wide variety of applications although care must be taken to prevent the delamination of the coated regions 506 as a result of thermal stress. To this end the silicon carbide paste preferably comprises a relatively coarse silicon carbide particulate, such as silicon carbide grinding powder, mixed with a suitable inorganic binder such as sodium silicate or aluminium cement. For certain applications however a quanitity of zirconia or zirconia mixture may be added to improve the high temperature susceptibility of the coated regions 506 and bring their thermal expansion coefficients closer to that of the substrate 502. In use the coated regions 506 are spaced apart so as to not only limit the magnitude of the thermal stresses in each region 506 but also maintain the integrity of the interface between the coated region 506 and the substrate 502 as the susceptor 500 shrinks or expands. The substrate .J2 on the other hand may be of fibreboard in either its low density (less than 300kg/m³) or high density (less than 700kg/m³) format or alternatively may comprise a high alumina firebrick which, by virtue of its greater mechanical strength and higher thermal conductivity, is better able to resist the high temperatures and stresses generated by the silicon carbide coated regions 506.

A further embodiment is illustrated in Figure 7 in which a susceptor 600 is shown to comprise an alumina rod 602 received within a recess 604 in a wall 606 of a microwave enclosure 608, the alumina rod 602 having a silicon carbide coating 610.

In use, as with the previous embodiment, the silicon carbide coating 610 is heated by the absorption of microwaves and this heat is subsequently radiated by the susceptor 600 to its surroundings. The presence of the alumina rod 602 not only provides the susceptor 600 with its mechanical strength but also contributes to the even distribution of heat.

It will be apparent to those skilled in the art that the embodiments shown in Figures 6 and 7 may be adapted for use with other alumina surfaces such as those of crucibles and container vessels or even those of the other susceptors such as the surfaces of the tubes shown in Figures 2 and 3.

In a proposed microwave furnace layout any or all of the susceptors shown in Figures 2 to 7 may be used in different combinations depending on the application. In one arrangement for example, the susceptors may be positioned around the perimeter of the microwave furnace with the primary object of maintaining the surfaces of the enclosure at an elevated temperature. This alone would permit faster heating rates to be achieved than are currently available using conventional furnaces not withstanding the volumetric contribution to the heating of the sample material. This heating rate may be increased still further if the susceptors were interspersed throughout the enclosure and positioned so as to pass between the components to be heated. In this way all the components in the furnace would be exposed to an even distribution of radiant heating so that the susceptibility to microwaves of a whole batch could be increased substantially uniformly. Furthermore, it has been found that by exposing a surface of each of the components to a degree of radiant heat it is possible to balance out the thermal stresses caused by the reverse temperature distribution characteristic of microwave heated materials in which the interior is hotter than the exterior. The even distribution of the susceptors throughout the furnace also provides the additional benefit of ensuring an efficient and balanced load to the magnetron even at low temperatures.

A typical three stage furnace profile is shown in Figure 8 and relates to the firing of an advanced ceramic. As can be seen, the component is first heated to a moderate temperature, typically less than 500°C, at a rate of l-5°C/minute in order to facilitate the controlled removal of binder additives without incurring the risk of cracking or porosity since any defects introduced into the ceramic at this stage may remain after firing and affect the final quality of the component. Having reached a temperature of between 500 and 600°C the component is considered free of volatile constituents and firing can commence. Using a heating rate of 10-50°C/minute the firing stage may be kept relatively short in order to limit grain growth within the ceramic. Having reached its peak temperature the component is simply allowed to achieve thermal equilibrium before then entering the third and final cooling stage. The rate at which the component is allowed to cool is dictated by the insulation characteristics of the furnace and the thermal mass of the ceramic but can be fairly lengthy if cracking as a result of thermal shock is to be avoided, especially within the critical sub 700°C temperature range.

Since the duration of each stage of the furnace profile may typically be in the ratio 10:1:10 it will be apparent that a microwave source capable of achieving the desired heating rate during the firing stage will only be operating at a small fraction of its capacity during the remainder of the cycle. An integrated batch processing system that takes advantage of this fact is shown in Figure 9.

The system 700 illustrated in Figure 9 comprises three furnaces 702, 704 and 706 each having their own dedicated low power microwave source 708. Above the furnaces 702, 704 and 706, and suspended from a gantry 710, a high power microwave source 712 is mounted by means of brackets 714 and runners 716 for movement between one of three positions overlying a respective one of the furnaces 702, 704 or 706. A waveguide 718 hangs from the high power microwave source 712 and may be detachably connected to any selected one of the furnaces 704.

Because microwave furnaces employ a waveguide or coaxial cable to transport microwaves from a microwave source to a microwave enclosure, the microwave source can be regarded as detachable and as a result may be shared between a number of furnaces. Thus the system 700 enables the firing of three batches of components to take place in little more than the time taken to fire a single batch but with a much improved efficiency as a result of using the dedicated low power microwave sources 708 to perform the binder burnout and cooling stages of the operation and only employing the high power microwave source 712 for the energy intensive firing stage. The low power microwave sources 708 need not even operate at the same microwave frequency as the high power source 712 but may operate at a higher frequency to obtain better coupling at low temperatures improving both their efficiency and their cost effectiveness.

The maximum number of furnaces that may be supported by a single high power microwave source is clearly limited by the ratio of the duration of the firing stage to that of the overall process. As previously mentioned, this ratio is typically 1:10 and accordingly up to ten furnaces may be supported in this way although this figure will of course vary depending on the material, shape and size of the components to be fired. Using the described integrated batch processing system the furnaces 702, 704 and 706 may be manufactured at a minimum cost and, as illustrated in Figure 10, may comprise an enclosure 720 formed from pressed or folded stainless steel, aluminium sheet or microwave opaque mesh. Advantageously the enclosure 720 may be lined with concentric layers of self supporting ceramic fibreboard 722 and 724 and provided with a flange (not shown) for coupling the furnace to the waveguide 718. One or more susceptors 726 may be disposed within the enclosure 720 close to the component to be fired 728 so that in use the component 728 is exposed to radiant heat.

Similarly a high power microwave source 712 for use in connection with the described integrated batch processing system is shown in Figure 11 to comprise a magnetron 730 provided with a motorised automatic stub tuner 732, a circulator 734 and various monitoring devices such as a pyrometer 736 and a back reflected power meter 738. Advantageously the plate 740 used for mounting these devices may also be used as the microwave coupling for a built in quarter wave choke 742 while a mode stirrer 744 driven by a motor 746 may be provided to improve field uniformity. In this way a large proportion of the capital cost of the system 700 may be included in a single microwave source 712 that can then be shared between several furnaces.

In an alternative integrated batch processing system shown in Figure 12 the dedicated low power microwave sources 708 of the preceding embodiment are replaced by a single generator of radiant heat for use during the initial binder burnout stage. The advantages inherent to radiant heaters in terms of their reduced operating and manufacturing costs, their effectiveness at low temperatures and their ability to prevent thermal runaway have already been described above although it will be apparent from that discussion that if the radiant heaters are to comprise resistive elements then such elements will need to be retractable and the furnaces into which they extend will need to be adapted accordingly.

Referring to Figure 12 the illustrated system 800 can be seen to comprise a number of batch furnaces 802 each of which is mounted for movement on one of three parallel processing lines 804, 806 or 808. A first gantry 810 disposed transverse to the processing lines 804, 806 and 808 is provided with a generator of radiant heat 812 comprising either a low power microwave source coupled to one or more susceptors or a plurality of resistive heating elements. In either event the generator of radiant heat 812 is suspended at an adjustable height from the first gantry 810 and in such a way as to be moveable between one of three positions overlying a respective one of the three processing lines 804, 806 or 808.

A second gantry 814 is similarly disposed transverse to the processing lines 804, 806 and 808 but unlike the first gantry 810 from which it is spaced, the second gantry 814 is provided with a high power microwave source 816. The high power microwave source 816 is suspended at an adjustable height from the second gantry 814 and in such a way as to be moveable between one of three positions overlying a respective one of the three processing lines 804, 806 or 808. A waveguide 818 hangs from the high power microwave source 816 to be detachably connected to any selected one of the furnaces 802. In use a furnace 802 is loaded onto one of the three processing lines 804, 806 or 808 and moves to a position directly underneath the generator of radiant heat 812. The radiant heat produced by the generator 812 is used to carry out the binder burnout stage of the firing sequence and when this has been completed the furnace 802 moves on to a position directly below the high power microwave source 816. The waveguide 818 is connected to the furnace 802 and the firing stage is commenced. When this stage has also been completed the waveguide 818 is detached from the furnace 802 and the furnace 802 moved to a further position in which it is allowed to cool in a controlled manner, possibly with the aid of a further low power microwave source (not shown) .

It will be apparent that by sharing between furnaces 802 not only a high power microwave source 816 but also a generator of radiant heat 812, further cost savings are possible over and above those of the system shown in Figure 9. Again there is a limit to the number of furnaces 802 that may be employed in any one such system and this is determined by the ratio of the duration of the firing stage to that of the binder burnout stage. This ratio is typically between 1:3 and 1:10 so that to achieve optimum efficiency between three and ten generators of radiant heat 812 are needed to support one high power microwave source 816.

In order to further enhance the efficiency of both the system shown in Figure 9 and the system shown in Figure 12 a control apparatus may be provided that would serve to calculate the optimum sequencing of furnaces, control the relative positioning of the furnaces and heating units, monitor and control the temperature within each furnace, handle a number of furnaces each having different loads and recognise changes in the load characteristics during processing. Means may also be provided to calculate an optimum furnace design for a specific load, including where appropriate any insulation requirements, and facilitate the generation of an optimum furnace profile based on a knowledge of component geometry, dielectric properties, sintering rate and thermal shock resistance.

From the foregoing it will be apparent to those skilled in the art that the integrated batch processing systems described possess significant advantages over conventional continuous furnaces in that the systems described permit throughputs comparable to similarly rated continuous systems to be achieved without any loss of process flexibility, enable a number of different materials to be processed using the same system without introducing problems of cross-contamination, are more energy efficient in the absence of a transport system having a high thermal mass, are more adaptable to changing processing requirements because of the interchangability of different furnace units, enable the integration of drying and calcining operations on the same processing line, and, most importantly of all, allow full exploitation of the advantages inherent to microwave processing.

Furthermore it will be apparent to those skilled in the art that the described systems also possess significant advantages over conventional batch furnaces in that in the systems described one high' power microwave source is able to replace the many such sources used in previous batch arrangements enabling much of the ancillary equipment such as temperature controllers and fume extractors to be removed. This, together with the constant load factor, enables a simiplification of the power supply lines and provides a system that is cheaper than conventional batch furnaces, may be easily automated and run for twenty-four hours a day with only minimal manpower and that does not become redundant simply by virtue of a change in the product to be processed.

Claims

1. A microwave furnace comprising an enclosure for the confinement of microwaves, means defining an aperture in a wall of the enclosure, and at least one radiant heating element moveable between a first position in which the heating element extends into the enclosure through the aperture and a second position in which the heating element is withdrawn from the enclosure.

2 . A microwave furnace in accordance with claim l, wherein said at least one radiant heating element comprises a resistive heating element.

3. A microwave furnace in accordance with claim 2, wherein said at least one radiant heating element comprises two coaxial quartz tubes with a length of resistive heating wire disposed therebetween.

4. A microwave furnace in accordancce with claim 3, wherein the resistive heating wire comprises a kanthal type heating wire.

5. A microwave furnace in accordance with claim 1, wherein said at least one radiant heating element comprises a microwave susceptor.

6. A microwave furnace in accordance with any preceding claim wherein a guide is provided to facilitate the movement of said at least one radiant heating element between said first and second positions. 7. A microwave furnace in accordance with any preceding claim, wherein the aperture has a diameter of less than 0.

7 of the wavelength of the microwaves to be confined in the enclosure.

8. A microwave furnace in accordance with any preceding claim, wherein said at least one radiant heating element is held by means of a transverse member capable of sliding relative to one or more supporting pillars that extend substantially perpendicularly to said wall of the enclosure.

9. A microwave furnace in accordance with any preceding claim, wherein there is further provided a temperature sensor moveable between a first position in which the sensor extends into the enclosure through an aperture provided in a wall thereof and a second position in which the sensor is withdrawn from the enclosure.

10. A microwave furnace substantially as herein described with reference to Figure 1 of the accompanying drawings.

11. A radiant heating element for use within a microwave furnace comprising a first material that is substantially susceptible to microwaves at ambient temperatures and a second material having a greater thermal conductivity than the first material in order to distribute the heat that in use is generated within the first material.

12. A radiant heating element in accordance with claim 11, wherein said first material is contained within a substantially cylindrical tube formed from said second material.

13. A radiant heating element in accordance with claim 12, wherein the tube defining the radiant heating element is adapted to be mounted by one end from a wall of the microwave furnace and closed adjacent said one end by a refractory plug.

14. A radiant heating element in accordance with claim 12, wherein the tube defining the radiant heating element is free standing within the microwave furnace and closed at opposite ends by respective refractory plugs.

15. A radiant heating element in accordance with claim 13 or claim 14, wherein the or each refractory plug is formed of refractory alumina, zirconia cement or ceramic fibre.

16.. A radiant heating element in accordance with claim 11, wherein said first material comprises a plurality of beads coaxially mounted on a rod formed from said second material.

17. A radiant heating element in accordance with claim 16, wherein pairs of adjacent beads are held in mutually spaced relationship by means of a spacer formed from said second material.

18. A radiant heating element in accordance with any of claims 11 to 17, wherein a surface of said second material is provided with a silicon carbide coating.

19. A radiant heating element in accordance with claim 18, wherein the silicon carbide coating comprises a plurality of spaced apart coated regions.

20. A radiant heating element in accordance with any of claims 11 to 19, wherein said first material comprises zirconia or a zirconia mixture.

21. A radiant heating element in accordance with any of claims 11 to 20, wherein said first material comprises powdered graphite.

22. A radiant heating element in accordance with any of claims 11 to 21, wherein said first material comprises silicon carbide.

23. A radiant heating element in accordance with any of claims 11 to 22, wherein said second material comprises alumina.

24. A microwave furnace incorporating a radiant heating element in accordance with any of claims 11 to 23.

25. A radiant heating element substantially as herein described with reference to any of figures 2,3,4,5,6 or 7 of the accompanying drawings.

26. A microwave furnace substantially as herein described with reference to any of figures 2,3,4,5,6 or 7 of the accompanying drawings.

27. A microwave furnace comprising an enclosure for the confinement of microwaves, means defining a hot zone within said enclosure and a plurality of radiant heating elements that extend through and intersperse the hot zone.

28. A microwave furnace in accordance with claim 27, wherein any one of said plurality of radiant heating elements is in accordance wtih any of claims 11 to 23.

29. A microwave furnace in accordance with claim 27, wherein any one of said plurality of radiant heating elements comprises a retractable resistive heating element.

30. A batch processing system comprising a plurality of microwave enclosures, means for providing each of the enclosures with low level heating, a high power microwave source and means for coupling the high power microwave source to any selected one of the enclosures.

31. A batch processing system in accordance with claim 30, wherein the means for providing each of the enclosures with low level heating comprises a plurality of dedicated low power heaters.

32. A batch processing system in accordance with claim 30, wherein the means for providing each of the enclosures with low level heating comprises a low level heater and means for coupling the low level heater to any selected one of the enclosures.

33. A batch processing system in accordance with claim 31 or claim 32 wherein the low power heater comprises a low power microwave source.

34. A batch processing sustem in accordance with claim 31 or claim 32 wherein the low power heater comprises a radiant heater.

35. A batch processing system in accordance with claim 34 wherein the radiant heater comprises a microwave susceptor.

36. A batch processing system in accordance with claim 34 wherein the radiant heater comprises a retractable resistive heating element.

37. A batch processing system substantially as herein described with reference to Figures 8 to 12 of the accompanying drawings.