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WO1993012629A1 - Appareil et procede de chauffage par micro-ondes - Google Patents

Appareil et procede de chauffage par micro-ondes Download PDF

Info

Publication number
WO1993012629A1
WO1993012629A1 PCT/GB1992/002277 GB9202277W WO9312629A1 WO 1993012629 A1 WO1993012629 A1 WO 1993012629A1 GB 9202277 W GB9202277 W GB 9202277W WO 9312629 A1 WO9312629 A1 WO 9312629A1
Authority
WO
WIPO (PCT)
Prior art keywords
thermal screen
ceramic material
enclosure
microwave energy
heating
Prior art date
Application number
PCT/GB1992/002277
Other languages
English (en)
Inventor
Alan Leslie Bowden
Michael Gower Hamlyn
Original Assignee
Staffordshire University Enterprises Limited
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Staffordshire University Enterprises Limited filed Critical Staffordshire University Enterprises Limited
Publication of WO1993012629A1 publication Critical patent/WO1993012629A1/fr

Links

Classifications

    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F27—FURNACES; KILNS; OVENS; RETORTS
    • F27D—DETAILS OR ACCESSORIES OF FURNACES, KILNS, OVENS OR RETORTS, IN SO FAR AS THEY ARE OF KINDS OCCURRING IN MORE THAN ONE KIND OF FURNACE
    • F27D99/00—Subject matter not provided for in other groups of this subclass
    • F27D99/0001—Heating elements or systems
    • F27D99/0006—Electric heating elements or system
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
    • H05B6/00—Heating by electric, magnetic or electromagnetic fields
    • H05B6/64—Heating using microwaves
    • H05B6/6447—Method of operation or details of the microwave heating apparatus related to the use of detectors or sensors
    • H05B6/645—Method of operation or details of the microwave heating apparatus related to the use of detectors or sensors using temperature sensors
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
    • H05B6/00—Heating by electric, magnetic or electromagnetic fields
    • H05B6/64—Heating using microwaves
    • H05B6/74—Mode transformers or mode stirrers
    • H05B6/745—Rotatable stirrers
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F27—FURNACES; KILNS; OVENS; RETORTS
    • F27D—DETAILS OR ACCESSORIES OF FURNACES, KILNS, OVENS OR RETORTS, IN SO FAR AS THEY ARE OF KINDS OCCURRING IN MORE THAN ONE KIND OF FURNACE
    • F27D99/00—Subject matter not provided for in other groups of this subclass
    • F27D99/0001—Heating elements or systems
    • F27D99/0006—Electric heating elements or system
    • F27D2099/0028—Microwave heating

Definitions

  • This invention relates to a microwave heating method and apparatus, particularly for sintering/firing ceramic materials such as pottery, industrial ceramics and the like, in large batches of an industrial scale.
  • Ceramic materials generally absorb little electromagnetic energy at microwave frequencies at room temperature. However at higher temperatures their ability to absorb microwave energy can increase considerably, as shown in Figure 1 which is a graph of power absorbed against temperature. This can cause rapid, localised heating in a ceramic piece, should the temperature within the piece be, or become, non-uniform. This is particularly true at temperatures above temperature 'A' in Figure 1. The phenomenon is known as 'thermal runaway' .
  • the degree of attenuation of power within a piece increases with power dissipation within the piece, to which it is mathematically related. Since power dissipation can increase significantly with temperature ( Figure 1) the depth within the piece in which significant power is generated (known as the depth of penetration) can decrease significantly. Should the depth of penetration decrease such that it becomes comparable with the dimensions of the piece then the processing time must be extended to allow equalisation of temperature throughout the piece by thermal conduction. A final, stable situation is reached when the power input becomes constant for a set temperature. In practice, the depth of penetration is normally much greater than the dimensions of ceramic pieces and thus the attenuation of the electric field is not a problem.
  • the object of the present invention is to overcome or at least mitigate all or some of the problems referred to.
  • a method of heating ceramic material in an interior of an enclosure comprising providing a variable power source of microwave energy, providing in said interior of the enclosure a thermal screen which is around the ceramic material, in use, and which is heated by said microwave energy, operating said source to supply microwave energy to said interior of said enclosure containing said ceramic material, in use, and continuously controlling the power of said source in response to a difference between a set value of a parameter of the system, and a measured or calculated value of the parameter during operation of the method.
  • the method includes passing heated gas/air into the interior of the enclosure and around the ceramic material.
  • the hot gas/air is circulated in a closed system containing a fan.
  • apparatus for heating ceramic material comprising an enclosure having an interior in which said ceramic material is intended to be received, a thermal screen in said interior around said ceramic material, in use, a variable power source of microwave energy outside the enclosure operable to supply microwave energy to said interior of the enclosure containing said ceramic material, in use, and means for continuously controlling the power of said source, in use, in response to a difference between a set value of a parameter of the system, and a measured or calculated value of the parameter during the heating.
  • Figure 1 is a graph showing a known relationship between absorption of microwave power and temperature for ceramic materials
  • Figure 2 is a block diagram of part of a system of the present invention for providing accurate control of temperature sufficient to prevent thermal runaway whilst heating ceramic materials by microwave energy,
  • Figure 3 is a graph showing the relationship between temperature and the depth of a ceramic article when the article is heated by a known microwave heating method
  • Figure 4 diagrammatically shows the stacking of ceramic articles to be heated by a method of the invention
  • Figure 5 diagrammatically shows the passing of heated gas through an enclosure according to the present invention
  • Figure 6 is a diagram showing the use of a thermal screen with the method of the present invention.
  • Figure 7 is a block diagram of the method of the present invention.
  • the phenomenon known as 'thermal runaway' can be a problem with the heating of ceramic materials by microwave energy. To prevent this problem, it is necessary to detect rapid increases of temperature or rate of energy absorption of the ceramic piece and correspondingly to reduce the rate of microwave energy input, or power, into the piece. Therefore continuous control of the microwave energy source is necessary to achieve uniform heating of the piece.
  • the arrangement shown in Figure 2 is an arrangement for preventing thermal runaway.
  • FIG. 2 Shown in Figure 2 is a basic system for sintering ceramic materials.
  • the values of frequency, power and volts shown are by way of example only.
  • the system has a microwave resonant cavity 10 within which is contained a refractory box 11 for containing therein the ceramic article or articles to be sintered.
  • a source of microwave energy such as a magnetron 12 is coupled to the cavity 10 directly, or indirectly such as by a waveguide 13.
  • One or more optical fibre probes 14 are inserted directly into the refractory box and placed near to or in contact with the surfaces of the piece or pieces whose temperature is to be measured.
  • the optical probes represent only one possible means of temperature measurement.
  • the desired control of the microwave energy source is achieved by an electronic circuit containing one or more solid state devices, and indicated at 15, the solid state device being, for example, a thyristor with its control circuit, the thyristor controlling the voltage supply indicated at 16, to the magnetron.
  • the control signal to the thyristor may be derived, as shown in Figure 2, from either the temperature of the piece or alternatively from the power output of the magnetron. It can be a directly measured value of the chosen parameter or could be calculated from the measured value of a different parameter.
  • the control signal is compared with a set temperature or power value and a difference or error signal, if present, is fed to the thyristor control. Since energy radiated by the magnetron generator is converted directly to heat energy within the ceramic piece, a change in energy generated by the magnetron causes an instantaneous change in energy absorbed by the piece.
  • the set temperature may be programmed by computer to follow a pre-set temperature-time profile or a power- time profile to obtain the required characteristic of the piece or pieces in the minimum time.
  • Figure 2 shows a temperature control system 18, which can incorporate a suitable computer.
  • the temperature of a piece is sensed by the optical fibre thermometry system.
  • Thermocouples are not suitable since they cannot be placed permanently within the cavity 10 or refractory box 11 because they are metallic.
  • Infra-red pyrometry can be used by viewing the piece from outside the cavity 10 through a port in the walls of the cavity and refractory box respectively.
  • power from the magnetron source is, in one example, fed into the cavity 10 through the waveguide 13.
  • the cavity is preferably multimodal, namely it will support a large number of resonant modes in a given frequency range, and in a preferred embodiment the cavity can have a mode stirrer 13a to provide greater uniformity of electric field distribution within the cavity.
  • an array of magnetrons may be required.
  • the magnetron effectively provides a variable power source of microwave energy, so that the temperature of the piece can be controlled to prevent thermal runaway.
  • Figure 4 shows one possible scheme for stacking individual ceramic items 19 in the refractory box 11. This is just one form of stacking where the pieces are so arranged that temperature gradients between the centre and edges of the pieces due to conduction and radiation are minimised.
  • Figure 5 another technique for reducing or preventing loss of heat energy is to pass hot air or other gas or gases through the refractory box 11 and around the ceramic pieces to assist the equalisation of temperature throughout the refractory box. This is particularly advantageous in establishing a uniform temperature up to temperature A in Figure 1. It is thus of great benefit at temperatures below approximately 850°C, but also assists the prevention of the loss of heat energy above this temperature.
  • the air or other gas or gases may be heated at 20 within a closed system containing a fan 21, the gases being continuously circulated through the refractory box 11 in which, in this example, to provide an even distribution of gas flow, there are spaced baffles 22 adjacent to the inlet and outlet of the refractory box.
  • the tubes 23 to and from the refractory box 11 are preferably ceramic tubes designed with microwave choke to prevent radiation leaks.
  • the baffles can also/alternatively serve to regulate the passage of gas into and out of the box.
  • the flow rate and gas temperature are preferably controlled.
  • FIG. 6 A third technique for preventing or reducing loss of heat energy is shown in Figure 6, where a batch of ceramic articles 19 to be sintered are surrounded by a thermal screen 24, which in this embodiment, is of reticulated structure and made of ceramic material which is an absorber (or susceptor) of microwave energy and is thus heated thereby.
  • the porous nature of the structure allows the use of the technique described in relation to Figure 5, namely the recirculation of heated air or gas, in conjunction with the use of this thermal screen within the box 11, which as shown in Figure 6, surrounds the screen.
  • the relative susceptibility of the screen and batch to microwave energy may be controlled by doping the ceramic material from which the screen is made by a predetermined amount of a known, good absorber, for example silicon carbide.
  • a known, good absorber for example silicon carbide.
  • the reticulated nature of the screen provides low thermal mass, thereby permitting rapid temperature responses to changes in microwave power, and as stated, also allows the system to be used with circulating hot air or gas.
  • the use of a heated screen is particularly effective at process temperatures greater than approximately 1,000°C, when heat loss by radiation becomes dominant with known methods.
  • the screen itself could be surrounded by suitable thermal insulation.
  • thermal screen may be formed by the doping or spray coating of thermally insulating material with a known susceptor (absorber) of microwave energy.
  • the thickness of the doped insulation or spray coating, the nature of the susceptor, and the density of the susceptor are selected to match the specific application required.
  • Spray coatings are particularly suited to the heating of larger, individual items of variable geometry. The coating thickness and susceptance in these cases may be varied over the surface of the insulation, being greatest where the ratio of surface area to volume of the piece is large.
  • the spray coating may be on stiff, flat board or on board shaped or profiled to match the shape or profile of the piece.
  • a variation of this spray coated, layered screen may be more simply obtained by constructing the screen as a composite of materials of differing susceptibility, in sheet or other form. Again these alternative forms of screen could be porous to allow the hot air or gas to be circulated through the refractory box 11.
  • the first and second techniques described above are optional, so that neither, one, or both can be used with the third technique, namely the provision of the thermal screen, which is used with the means and method of Figure 2.
  • the material of the screen may be so formed as to provide a closed electrical circuit.
  • the circuit may be connected to a voltage and current source 24a outside the cavity which can be independently controlled, or, preferably, controlled in conjunction with and in correct relation to the controlled microwave energy source.
  • Figure 7 is a schematic, block diagram of a complete integrated control system for an embodiment of the method of the invention. Reference numerals already referred to have been used in this figure for the same parts.
  • the refractory box 11 has fed to it through tubes 23 the heated gas or air referred to above, this being passed through a heater 20 and fan 21.
  • Indicated at 26 is an integrated digital control system and connected to this are lines 27, 28 respectively providing values of the temperature and flow rate of the gas or air fed to the refractory box.
  • lines 29, 28 respectively providing values of the temperature and flow rate of the gas or air fed to the refractory box.
  • lines 29 and 30 respectively which feed values of the screen temperature and the batch temperature from the refractory box.
  • a line 31 extends from the control system 26 and carries a signal to the control circuit of the magnetron to vary its power as required, although this as shown in Figure 7 goes directly to the cavity 10, since the variation in power and thus of microwave energy is transmitted into the microwave cavity.
  • Line 31 also carries a signal to the external, independent, power supply 24a to the screen, if its conductive nature is employed.
  • the comparable parameter for regulating the power of the magnetron could be its own measured or calculated power value, rather than the temperature of the batch or ceramic piece being heated.
  • the heated air or gas is undesirable when firing glaze on to whiteware articles.
  • the controls (a) and (b) mentioned above can be integrated and matched to achieve the required material properties/microstructure of the piece or pieces.
  • Optimum control of the total scheme is achieved by control of the variable parameters within (b) by a multi- input, adaptive/self-tuning digital control scheme.
  • Control of gas flow and its heating external to the cavity enables a relative temperature-time characteristics of the thermal screen and batch to be controlled.
  • the control scheme may include or be assisted by input data on the loss factor of ceramic materials at elevated temperatures.
  • the use of circulating hot air/gas is normally of great benefit at process temperatures lower than approximately 850°C, and in an embodiment using all three techniques, the stacking arrangement shown in Figure 4 would be used in conjunction with the use of the thermal screen and the circulation of the heated air/gas. At temperatures above approximately 1,000 3 C, the stacking system of Figure 4 could be used with the thermal screen of Figure 6 without the use of the heated gas/air.
  • thermal screen and hot gas forms a hybrid system of heating controlled by an integrated controller which compares measured values with set or computed values from a mathematical model of the system.
  • the heating of the thermal screen can be by virtue of the microwave energy alone or with the electrical heating.
  • the method described is preferably continuously controlled and the integrated control of all the variables prevents thermal runaway and enables uniform heating to be achieved within the ceramic piece and throughout the refractory box.
  • the method can thus enhance the uniformity, consistency and speed of the microwave heating and firing of the piece.

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  • Engineering & Computer Science (AREA)
  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Power Engineering (AREA)
  • Mechanical Engineering (AREA)
  • General Engineering & Computer Science (AREA)
  • Constitution Of High-Frequency Heating (AREA)
  • Furnace Details (AREA)

Abstract

Un appareil pour chauffer des articles (19) en matériau céramique par l'énergie de micro-ondes comprend une cavité de résonance à micro-ondes (10), une boîte en matériau réfractaire (11) à l'intérieur de la cavité pour recevoir les articles (19), un magnétron (12) pour produire des micro-ondes, un écran thermique (24) disposé autour des articles (19) pour être chauffé par l'énergie des micro-ondes, et des moyens (15-18) pour contrôler d'une manière continue la puissance du magnétron (12) en réponse à une quelconque différence entre une valeur fixée de la température des articles et une valeur mesurée de celle-ci. On peut faire circuler le gaz chauffé à travers l'écran et autour des articles pendant le processus de chauffage par micro-ondes, et on ajuste la sensibilité de l'écran thermique aux micro-ondes en le dopant ou en lui appliquant par pulvérisation des agents absorbant les micro-ondes. Dans une forme d'exécution, l'écran thermique peut également être chauffé électriquement.
PCT/GB1992/002277 1991-12-13 1992-12-09 Appareil et procede de chauffage par micro-ondes WO1993012629A1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB9126560.3 1991-12-13
GB919126560A GB9126560D0 (en) 1991-12-13 1991-12-13 Microwave heating method and apparatus

Publications (1)

Publication Number Publication Date
WO1993012629A1 true WO1993012629A1 (fr) 1993-06-24

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ID=10706257

Family Applications (1)

Application Number Title Priority Date Filing Date
PCT/GB1992/002277 WO1993012629A1 (fr) 1991-12-13 1992-12-09 Appareil et procede de chauffage par micro-ondes

Country Status (3)

Country Link
AU (1) AU3090992A (fr)
GB (2) GB9126560D0 (fr)
WO (1) WO1993012629A1 (fr)

Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2332983A (en) * 1997-12-31 1999-07-07 Samsung Electronics Co Ltd Forming a thin film on a wafer using microwave heating
WO2001005191A1 (fr) 1999-07-07 2001-01-18 Corning Incorporated Sechage par micro-ondes de ceramiques
US6344634B2 (en) 1999-12-28 2002-02-05 Corning Incorporated Hybrid method for firing of ceramics
US6344635B2 (en) 1999-12-28 2002-02-05 Corning Incorporated Hybrid method for firing of ceramics
US6537481B2 (en) 1999-12-28 2003-03-25 Corning Incorporated Hybrid method for firing of ceramics
US6706233B2 (en) 2000-12-29 2004-03-16 Corning Incorporated Method for processing ceramics using electromagnetic energy
US11254071B2 (en) * 2016-10-31 2022-02-22 Beijing Goldwind Science & Creation Windpower Equipment Co., Ltd. Flexible molding process and system for magnetic pole protective layer

Families Citing this family (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2281016A (en) * 1993-08-10 1995-02-15 Ea Tech Ltd Microwave-assisted processing of materials
DE19642673A1 (de) * 1996-10-16 1998-04-23 Widia Gmbh Mikrowellenofen und Bauteile hierfür
DE19648366C1 (de) * 1996-11-22 1998-04-02 Riedhammer Gmbh Co Kg Anlage zur thermischen Behandlung von Produkten
IT1304897B1 (it) * 1998-09-08 2001-04-05 Gaudenzio Borelli Forno a microonde per la cottura di materiale ceramico sotto pressione
JP2003075077A (ja) * 2001-09-05 2003-03-12 Natl Inst For Fusion Science マイクロ波焼成炉およびマイクロ波焼成方法
JP2006518445A (ja) * 2003-02-10 2006-08-10 ビーティーユー インターナショナル インコーポレイテッド 材料の均一加熱処理の方法とそのシステム
DE102012215263A1 (de) * 2012-08-28 2014-03-06 BSH Bosch und Siemens Hausgeräte GmbH Gargerät
CN105666895B (zh) * 2016-01-15 2017-09-15 中南大学 一种复合能场加热装置

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Publication number Priority date Publication date Assignee Title
US4307277A (en) * 1978-08-03 1981-12-22 Mitsubishi Denki Kabushiki Kaisha Microwave heating oven
US4822966A (en) * 1987-02-20 1989-04-18 Yuzuru Matsubara Method of producing heat with microwaves
US4880578A (en) * 1988-08-08 1989-11-14 The United States Of America As Represented By The United States Department Of Energy Method for heat treating and sintering metal oxides with microwave radiation
GB2227397A (en) * 1989-01-18 1990-07-25 Cem Corp Microwave ashing furnace
EP0412019A1 (fr) * 1989-08-04 1991-02-06 Regie Nationale Des Usines Renault Dispositif d'élimination des particules carbonées contenues dans les gaz d'échappement de moteurs thermiques

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Publication number Priority date Publication date Assignee Title
US4416623A (en) * 1982-02-01 1983-11-22 Kanto Yakin Kogyo Kabushiki Kaisha Muffle furnace
JPS5991594U (ja) * 1982-12-09 1984-06-21 ハウス食品工業株式会社 マイクロ波乾燥装置

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4307277A (en) * 1978-08-03 1981-12-22 Mitsubishi Denki Kabushiki Kaisha Microwave heating oven
US4822966A (en) * 1987-02-20 1989-04-18 Yuzuru Matsubara Method of producing heat with microwaves
US4880578A (en) * 1988-08-08 1989-11-14 The United States Of America As Represented By The United States Department Of Energy Method for heat treating and sintering metal oxides with microwave radiation
GB2227397A (en) * 1989-01-18 1990-07-25 Cem Corp Microwave ashing furnace
EP0412019A1 (fr) * 1989-08-04 1991-02-06 Regie Nationale Des Usines Renault Dispositif d'élimination des particules carbonées contenues dans les gaz d'échappement de moteurs thermiques

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
PATENT ABSTRACTS OF JAPAN vol. 15, no. 229 (E-1076)11 June 1991 *

Cited By (10)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2332983A (en) * 1997-12-31 1999-07-07 Samsung Electronics Co Ltd Forming a thin film on a wafer using microwave heating
GB2332983B (en) * 1997-12-31 2000-06-21 Samsung Electronics Co Ltd Apparatus for forming thin film using microwave and method therefor
US6384390B1 (en) 1997-12-31 2002-05-07 Samsung Electronics Co., Ltd. Apparatus for forming thin film using microwave and method therefor
WO2001005191A1 (fr) 1999-07-07 2001-01-18 Corning Incorporated Sechage par micro-ondes de ceramiques
US6259078B1 (en) 1999-07-07 2001-07-10 Corning Incorporated Method for microwave drying of ceramics
US6344634B2 (en) 1999-12-28 2002-02-05 Corning Incorporated Hybrid method for firing of ceramics
US6344635B2 (en) 1999-12-28 2002-02-05 Corning Incorporated Hybrid method for firing of ceramics
US6537481B2 (en) 1999-12-28 2003-03-25 Corning Incorporated Hybrid method for firing of ceramics
US6706233B2 (en) 2000-12-29 2004-03-16 Corning Incorporated Method for processing ceramics using electromagnetic energy
US11254071B2 (en) * 2016-10-31 2022-02-22 Beijing Goldwind Science & Creation Windpower Equipment Co., Ltd. Flexible molding process and system for magnetic pole protective layer

Also Published As

Publication number Publication date
GB9225749D0 (en) 1993-02-03
GB2262333A (en) 1993-06-16
AU3090992A (en) 1993-07-19
GB9126560D0 (en) 1992-02-12

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