Title of Invention

A MICROWAVE OVEN INTEGRATED WITH A CONTINUOUS PROCESS CONTROL DEVICE

Abstract

This invention relates to a microwave oven integrated with a continuous process control device adapted for use as a sintering furnace for small size samples, the oven comprising a kitchen microwave oven (A) with an oven cavity (7) having a main control unit (1) magnetron (2), wave guide (3) and a specimen (8) placed inside a casket covered at sides by porous zirconia plate (a) and SiC plate (4) and top and bottom surface of said enclosure covered by insulation material (5) and a thermocouple (6) inserted from the top to be in contact with the said specimen (8) and a power transformer (10) provided at the input to the primary of the magnetron (2). Characterized in that a continuous process control device in the form of a circuit comprising a contactor (12) and an electronic repeat-cycle timer (13), the circuit device being interposed at the primary of the power transformer (10) of the magnetron, and the said circuit of contactor (12) and timer (13) being provided with the pairs of NC contacts (C5-C7, C6-C8) which are connected to a thermocouple (Tc) and a temperature indicator (Tl) to monitor the sample temperature during microwave 'off conditions. Figs. 1 & 4

Full Text

This invention relates to a continuous process control device for a kitchen microwave oven for use as a sintering furnace for sintering small size samples, particularly upto a temperature of 1600°C. BACKGROUND OF THE INVENTION Microwaves are a form of electromagnetic energy characterized by mutually perpendicular electrical 'E' and magnetic 'H' fields. The FCC has reserved 915 MHz and 2450 MHz, among other frequencies, for industrial applications. At present, most microwave ovens in use operate at 2450 MHz, which is a wavelength of 4.8" in air. Materials differ in their reaction to microwave fields. Polar molecules in receptive materials respond to these fields by oscillating in rotary motion. The energy generated by this motion causes these substances to heat. Microwave energy has been in use for over 50 years in a variety of applications, such as communications, food processing, rubber volcanization, textile and wood products, and drying of ceramic powders. The application of microwaves in sintering of ceramics is relatively new. Although many potential advantages of using microwaves to process ceramics have been long recognized, it is only now that this field has finally shown to be at the take off stage, especially for the commercialization of some speciality ceramics, including composites. MICROWAVE SINTERING Firing or sintering, one of the most critical stages of ceramic manufacture, must be precisely controlled to avoid thermal stresses developing in the product. If this is not achieved it could result in failure of the piece or batch being fired. Microwave sintering employs microwaves to fuse powders into solids, which produce dense products with better mechanical properties. The Electro magnetic energy of the wave is efficiently converted into thermal energy helping to produce a finer grain size in the finished product than is produced through traditional sintering. Microwave sintering can be successfully used on a range of materials including metal powders. Advantages of the technique include volumetric heating, significantly faster heating rates, lower sintering temperatures, enhanced densification, and smaller average grain sizes. Basic interaction mechanisms are shown to depend strongly on the dielectric and magnetic properties of a process material. MICROWAVE SINTERING SYSTEM The microwave sintering apparatus consists of a kitchen microwave unit, a high temperature insulation arrangement for sample location, a port for temperature measurement. Microwave heating of ceramics produces a "cold-wall" furnace, since the sample is actually the heating element. Without appropriate insulation, parts sintered at 2.45 GHz typically sinter non-uniformly, fail to reach required sintering temperatures and/or crack badly. The insulation package termed as "casketing" and its arrangement are important. The casketing is made by placing Silicon Carbide (SiC) plates in a alumina fibre insulation. At low temperatures (less than 800°C) the main heating mechanism is the blackbody radiation, which emerges from the Susceptor SiC. Once the samples are heated to 800°C they start absorbing microwave independently and the sample temperature increases. Platinum-Rhodium thermocouple is inserted through a port for monitoring the temperature of the sample. This port can also be used as a view port if infrared temperature measurement system is used. The micro oven used for sintering studies was of power level 1 kW (as per IE standard) and has a cavity dimension of 35 x 21 x 35 cms. The control system is a micro processor based control with a timer and has a provision to vary average power in discrete steps of 10%, 30%, 50%, 70% and 100 % using simple on-off control. PRIOR ART The kitchen microwave oven commonly operates from a 230 volt A.C. power supply and can deliver up to lkW power output for heating up food items. Control over heat input is exercised in the oven through a touch control panel and a key unit. Operator-inputs through the key unit are conveyed to the touch control panel consisting of Large Scale Integration (LSI) integrated Circuits (ICS), etc. The controls required,as far as the normal utility of the oven is concerned, are rather simple and the requirements are pre-programmed and stored in the LSI IC. However, in order to achieve the critical power control required to attain the required heating rate, soaking at a specific temperature, etc it is necessary to get a greater flexibility in control functions and this will be attainable in the oven only by altering the software of the central processing unit (CPU). Even after such an alteration, to be able to get programs covering a large range so that case-to-case selections can be made for a particular sintering, the system will become complicated. Director of Microwave Research Centre, (MCRC) Penn State University , USA in his communication with our group has indicated a method used by them to circumvent this problem and obtaining a simple output power control by using a continuously variable auto transformer or variac. The variac is introduced at the input to the primary of the magnetron transformer so that input voltages to the magnetron, which is the source of microwaves, can be regulated to chosen values by the user. The kitchen microwave circuit has been modified with the inclusion of the variac. By varying the input voltage to the transformer upto the full value,, the heat input to specimens that are to be sintered is regulated and can be varied continuously at the discretion of the operator. There are disadvantages associated with the above microwave sintering system. The main disadvantage is that a minimum triggering voltage of 160 V for the magnetron is essential. Power output varies non-linearly for an input variation °f 170V-230V at the input of magnetron. Another disadvantage associated with the present system is that the water load test to estimate the power level showed that the efficiency was low and a peak power of 800W was attainable. This could be attributed to the varying input voltage applied to the heating element of the magnetron. SUMMARY OF THE INVENTION Therefore the main object of the present invention is to provide a continuous process control for a kitchen microwave oven for use as a sintering furnace. Another object of the present invention is to provide a control for varying the heating rate of the sample as well as holding the sample at the required temperature. Yet another object of the present invention is to provide a control which has the provision to measure temperature at preset intermittent time scale after switching off microwave for the said duration of measurement. This normally is of the order of 10 seconds. Still another object of the present invention is to provide facilities to transmit signal of the temperature sensor only during the microwave power off situation . According to the present invention there is provided a microwave oven integrated with a continuous process control device adapted for use as a sintering furnace for small size samples, the oven comprising a kitchen microwave oven (A) with an oven cavity having a main control unit magnetron, wave guide and a specimen placed inside a casket covered at skies by porous zirconia plate (a) and SiC plate and top and bottom surface of said enclosure covered by insulation material and a thermocouple mserted from the top to be in contact with the said specimen and a power transformer provided at the. input to the primary o£ the magnetron. Characterized in that/f continuous process control deviceAin the form of a circuit comprising a contactor and an electronic repeat-cycle timer, the circuit device being interposed at the primary of the power transformer of the magnetron, and the said circuit of contactor and timer being provided with the pairs of NC contacts (C5-C7, C6-C8) which are connected to a thermocouple (Tc) and a temperature indicator (Tl) to monitor the sample temperature during microwave 'off conditions. The nature of the invention, its objective and further advantages residing in the same will be apparent from the following description made with reference to the non-limiiing exemplary embodiments of the invention represented in the accompanying drawings : Figure 1 an arrangement of a sintering system built around a kitchen microwave oven according to the prior art; Figure 2 a schematic diagram of control for a conventional kitchen microwave oven; Figure 3 a schematic diagram of the microwave power control using variac interfaced to main control according to an improved prior art; Figure 4 a schematic diagram of the microwave power control using continuous control according to the invention; Figure 5 shows a graph of temperature/time (temperature profile) for a typical casket during sintering. DETAIL DESCRIPTION OF THE INVENTION In accordance with the present invention in which a kitchen microwave unit is used as a sintering apparatus with an improved control system. An arrangement of a sintering system built around a kitchen mcirowave oven (A) has been shown in Figure 1. The main control unit (1) and the magnetron (2) is shown outside the oven cavity (7). The magnetron (2) is connected to the oven cavity (7) with a wave guide (3). The specimen (8) is placed in a casket made of insulation material (5) and covered at sides with porous zirconia plate (9) and SiC plate (4). A thermocouple(6£nters the casket from the top of the microwave oven and is in contact with the specimen (8). The circuit diagram of the control for conventional kitchen microwave oven is shown in Figure 2. The main control unit (1) and the magnetron (2) have been identified in Figure 2. The circuit diagram of the mcirowave power control using variac interfaced to main control is shown in figure 3 which is used in the prior art. The variac unit (11) alogwith relay timer and NC contacts connected to a thermo couple (TC) and a temperature indicator (Tl). The addition of variac unit (11) indicated as dotted block ' B' is introduced at the input to the primary of the of the magnetron (2) (-2-) transformer (IG) SO that the input voltages to the magnetron (2) which is the source of micowaves can be regulated to chosen values by the user. In accordance with the present invention, embodied and broadly described herein is a device for continuous control of microwave sintering system built around a kitchen microwave even. Compared to the continuously variable auto-transformer (11) that has been introduced as in Figure 3 described in the prior state of art in the present preferred embodiment, a power contactor [C] (12) operated by an electronic repeat-cycle timer [T] (13) is included in the circuit dotted block 'C' which is shown in Figure 4. The contactor [C] (12) with a multiplicity of contacts is used to provide 'on-off' for the 230 V power supply to the primary [P] of the magnetron (2) transformer [Tr] (10). Two pairs of 'normally-open' contacts [C1-C3, C2-C4] carry out this control function by connecting or disconnecting the input power. The power to the contactor coil [CC] for operating the contactor is connected at terminals [C9-C10] through contacts [15,16] or [15,18] of the electronic repeat cycle timer [T]. AS the input A.C. voltage is switched on at XX', the timer [T] (13) get the power. The contacts switch from [15,16] to [15,18] and the contactor closes and establishes full power supply at 230 V to the primary [P] of the magnetron (2) transformer[Tr] (10). The timer (13) also starts the preset 'On time' simultaneously and the normally closed contacts [15-16] remain open through out this period. At the end of the 'On time' contacts [15-16] closes thereby cutting off power to the mangnetron (2) and therefore the heat input to the specimen (8) under process. The timer (13) meanwhile starts the 'Off time' at the end of which the timer contacts [15-18] once again close restoring power to the.primary (P) of the transformer (10).of the magnetron The timer (13) being a repeat-cycle type, the whole sequence repeats at a frequency decided by the preset on-off time. A wide range of settings is possible in the timer [T] (13) for both 'On' and 'Off times and this feature permits the operator to control the duty cycle of the power input, the input voltage remaining constant all the time. The heating filament supply of the magnetron (2) therefore gets its continuous rated supply voltage of 230 V and keeps the microwave source efficient. Depending on the nature of load to be heated,and the rate at which heating needs to be given,the operator can make a simple selection of 'On', and 'Off' times and in this manner control the heat cycles, the circuit block (C) being independent of the limited features provided by the built-in touch panel in the main control unit (1) of the kitchen microwave oven. In addition, the preferred embodiment also has two pairs of NC contacts (12) [C5- C7 & C6-C8] which serve to connect a thermocouple (6) [Tc] to monitor the sample (8) temperature. The thermocouple (6) is normally connected to a temperature indicating instrument [Tl] through these contacts. During times when the power contactor is in 'Off' condition,NC contacts (12) are closed thereby facilitating recording of temperatures. As and when the contactor (12) coil is energized these pairs of contacts open first by which action the measurement circuit of the thermocouple (6) with the display unit gets disconnected. Therefore the measurement cycle is complimentary to the 'Power 'on cycle ensxuring by which provision a means of safely recording temperature without any dangerous voltages coupling with the measurment circuit and resulting in a safety hazard. The timer [T] (13) and contactor [C] (12) being very simple inexpensive and easy to set up and operate therefore form a very useful and simple means of converting the kitchen microwave oven of which power and heat input can be controlled over an extended range. Further the whole system can be inserted into the oven circuit with little effort is easily operable and at the same time lends itself to temperature measurements without causing unsafe flash-over at the metallic wires connected for measurement. In the currently preferred embodiment the microwave cavity (7) has a dimension of 350 mm W. 350 mm D and 210 mm H. The casket used has a dimension of 130 mm OD x 70 ID and 140mm long tube formed by Alumina fiber mat. End mats were also cut to a minimum thickness of 3 Cms. The tubular shape was maintained by wrapping with glass cloth ribbons. SiC plates of 35x35x3 mm plates were used to form a rectangular container with porous zirconia plates as a separator on either side. This in turn provides a sample volume of 30 x30x30 mm inside the casket. The sample is placed in the casket and is located at the centerof the cavity. A hole is drilled from the top for using as a view port or for inserting thermocouple for measurement of temperature. Using Platinum-Rhodium thermocouple and making mesurement while microwave power is off control of the process which has been achieved in this embodiment. With platinum sheathing tube one can monitor temperature continuously without microwave signal pickup [Figure 1]. Very inexpensive neon lamps of 5mm long are used at various joints and openings to detect any microwave leakage as these lamps start glowing even if they are exposed to low power micro wave radiation.The continuous microwave power control scheme is interfaced by internally disconnecting the input leads of the magnetron (2) and routing it through the timer [13]. The thermocouple (6)leads are also routed through contactor (12) of the scheme so as to prevent microwave power reaching the measurement system when microwave is on. The on time of the timer is adjusted based on the heating rate required and also reduced in proportion to off time set for . measurement during holding of sample temperature. This furnace arrangement is capable of achieving temperatures in excess of 1600 °C. In particular no arcing has been observed either between the temperature sensor leads and the walls of the microwave cavity (7). The constancy of temperature depends on the casket arrangement and a variation less than _+l degree has been achieved. This should be analyzed in the light of a heating rate of 100-150 degrees/min. in samples under lkW microwave radiation.The temperature profile obtained in a typical cavity under the sintering condition is as shown in Figure 5. This reflects instantaneous and high degree of control attainable by the scheme disclosed in this invention. The invention described hereinabove is in relation to the non-limiting embodiments and as defined by the accompanying claims. WE CLAIM: 1. A microwave oven integrated with a continuous process control device adapted for use as a sintering furnace for small size samples, the oven comprising a kitchen microwave oven (A) with an oven cavity (7) having a main control unit (1) magnetron (2), wave guide (3) and a specimen (8) placed inside a casket covered at sides by porous zirconia plate (a) and SiC plate (4) and top and bottom surface of said enclosure covered by insulation material (5) and a thermocouple (6) inserted from the top to be in contact with the said specimen (8) and a power transformer (10) provided at the input to the primary of the magnetron (2), Characterized in that the said continuous process control device in the form of a circuit comprising a contactor (12) and an electronic repeat-cycle timer (13), the circuit device being interposed at the primary of the power transformer (10) of the magnetron, and the said circuit of contactor (12) and timer (13) being provided with the pairs of NC contacts (C5-C7, C6-C8) which are connected to a thermocouple (Tc) and a temperature indicator (Tl) to monitor the sample temperature during microwave 'off conditions. 2. A microwave oven integrated with a continuous process control device claimed in claim 1 wherein the microwave oven (A) for sintering small size specimen (8) upto a temperature of 1600OC is of power level of lkW. 3. A microwave oven integrated with a continuous process control device as claimed in claims 1 or 2 wherein said microwave oven (A) has a cavity (7) dimension of 35x21x35 cms. 4. A microwave oven integrated with a continuous process control device as claimed in claim 1 wherein the contactor [C] (12) with a plurality of contacts (C1-C10) is connected to provide power supply to the primary [P] of the power transform (10) of the magnetron (2) 5. A microwave oven integrated with a continuous process control device as claimed in claim 4 wherein the contacts [15,16] or [15,18] of the electronic repeat cycle timer [T] (13) is connected at terminals [C9, CIO] of the contactor coil [CC] (12) for operating the contactor [C] (12), 6. A microwave oven integrated with a continuous process control device as claimed in claim 5 wherein the input AC voltage is switched on at XX' the timer [T] (13) gets power when the contacts [15, 16] switches to [15,18] to close the contactor [C] (12) to connect to the primary [P] of the power transformer (10) for supply of power, 7. A microwave oven integrated with a continuous process control device as claimed in claim 5 or 6 wherein the timer (13) at the end of preset on time switches to contact [15,16] and cuts off power to the primary (P) of the power transformer (10) of the magnetron (2). 8. A microwave oven integrated with a continuous process control device as claimed in claims 5 to 7 wherein the timer (13) is a repeat-cycle type in which on and off timers can be set independently. 9. A microwave oven integrated with a continuous process control device as claimed in claim 1 wherein said contactor (12) is provided with NC contacts [C5-C7; C6-C8] and connected to a thermocouple [Tc] (16), said thermocouple (6) is connected to a temperature mdicator [Tl] facilitating temperature measurements during microwave off time. 10. A microwave oven integrated with a continuous process control device as claimed in claim 9 wherein said thermocouple (6) is a platinum-rhodium thermocouple, 11. A microwave oven integrated with a continuous process control device as claimed in claim 1 wherein the said circuit can be easily interfaced to the existing front panel control of a kitchen microwave oven. 12. A microwave oven integrated with a continuous process control device adapted for use as a sintering furnace for small size samples as herein described and illustrated with the accompanying drawings.

Documents:

1016-del-2001-abstract.pdf

1016-del-2001-claims.pdf

1016-del-2001-complete specification (granded).pdf

1016-del-2001-correspondence-others.pdf

1016-del-2001-correspondence-po.pdf

1016-del-2001-description (complete).pdf

1016-del-2001-drawings.pdf

1016-del-2001-form-1.pdf

1016-del-2001-form-19.pdf

1016-del-2001-form-2.pdf

1016-del-2001-form-3.pdf

1016-del-2001-gpa.pdf

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