JP7622920B2 - Heating device and heating method for heating a heating element - Google Patents

Description

Application of Article 30, Paragraph 2 of the Patent Act Presented at the 79th Autumn Meeting of the Japan Society of Applied Physics in 2018

Application of Article 30, Paragraph 2 of the Patent Law Presented at the 25th International Workshop AM-FPD18

The present invention relates to a heating device that uses microwaves, and in particular to temperature control of a heating element that absorbs microwaves and generates heat.

A microwave-based furnace is known as a furnace for firing metals, ceramics, etc. In this furnace, a heating element that absorbs microwaves and generates heat is applied to the inner wall of a container made of insulating material. An object to be heated containing metal powder or ceramic powder is then placed in the container, and the container is placed in a microwave oven for a predetermined time to be fired (see Patent Document 1). For example, a mixture of two types of highly heat-resistant silicon carbide powder and a heat-resistant material such as water glass or clay is used as the heating element.

JP 2001-284039 A

Because shortening the heating process time of the heating element results in rapid baking of the heated object in a short time, it is necessary to use microwaves to raise the temperature of the heating element, which serves as the heat source, as quickly as possible. However, in the above-mentioned microwave heating device in which the heating element is applied to the inner wall of the container, it is difficult to achieve a faster temperature rise than ever before because the heating element contains silicon, which has a high electrical resistance. Also, in order to achieve rapid baking in a short time, it is necessary to raise the temperature of the heating element quickly and ensure that the temperature of the heating element reaches the desired temperature without temperature disturbance.

Therefore, there is a demand for a heating device and a heating method that can achieve rapid heating of the heating element while precisely controlling the temperature.

The heating device of the present invention includes a heating element having a heat generating material containing carbon that absorbs microwaves and generates heat, and a container that is permeable to microwaves and in which the heat generating material is placed, and a temperature measuring unit that measures the temperature of the heating element. For example, the container can be filled with a heat generating material of carbon powder. In this case, at least one of the total thickness of the heat generating material and the packing rate of the powder is determined so that the temperature rise due to microwave absorption is dominant over the temperature rise due to thermal conduction between particles. Also, a fluid that is inert to the heat generating material can be enclosed in the container.

In the present invention, the heating device heats the heating element at a heating rate of 30 (°C/sec) or more, and irradiates the heating element with microwaves based on the measured temperature so as to maintain the temperature of the heating element at the target temperature. A heating rate of 30 (°C/sec) or more here refers to a heating rate measured temporarily while the temperature is rising, and represents the heating rate during the period when the temperature is rising at a nearly constant heating rate.

For example, the heating device can heat the heating element to a temperature close to the target temperature at a heating rate of 30 (°C/sec) or more, and then heat the heating element so that it asymptotically approaches the target temperature. On the other hand, the heating device can also cool the heating element at a cooling rate of 30 (°C/sec) or more.

The heating device can be configured to include a magnetron that irradiates microwaves to the heating element, and a control unit that controls the power of the magnetron. The control unit can control the power of the magnetron to heat the heating element at a heating rate of 30 (°C/sec) or more. For example, the power to the magnetron is 100 W or more, and the target temperature is set to 800°C or more.

The control unit can gradually increase the power of the magnetron from the start of operation until the target temperature is reached. Then, the power can be reduced as the heating element reaches a temperature close to the target, and temperature control can be performed by repeatedly increasing and decreasing the power multiple times. On the other hand, the control unit can gradually reduce the power when lowering the temperature of the heating element at a temperature reduction rate of 30 (°C/sec) or more.

Another aspect of the present invention is a heating method in which microwaves are irradiated to a heating element that includes a heat generating material containing carbon that absorbs microwaves and generates heat, and a container that transmits microwaves and contains the heat generating material. The method measures the temperature of the heating element, raises the temperature of the heating element at a heating rate of 30°C/sec or more, adjusts the temperature of the heating element to a target temperature, and irradiates the heating element with microwaves based on the measured temperature so as to maintain the temperature of the heating element at the target temperature.

According to the present invention, the heating element can be heated to the desired temperature in a short time, and the object to be heated can be heated.

FIG. 1 is a schematic diagram of a microwave heating device. 1 is a schematic cross-sectional view of a heating tube according to a first embodiment. FIG. 5 is a schematic configuration diagram of a microwave heating device according to a second embodiment. 1 is a graph showing the power of a microwave generator associated with microwave heating over time. 1 is a graph showing the temperature of a heating element over time due to microwave heating. FIG. 4 is a control block diagram relating to temperature control of a heating element.

Below, an embodiment of the present invention will be described with reference to the drawings.

Figure 1 is a schematic diagram of a microwave heating device according to a first embodiment. Figure 2 is a schematic cross-sectional view of a heating tube.

The microwave multiple reflection type heating device (also called a microwave baking furnace; hereafter referred to as the "microwave heating device") 10 is a device that uses microwaves to heat, bake or dry objects to be heated, and is equipped with a heat-resistant rectangular microwave reflecting container (hereafter referred to as the reflecting container) 40. Microwave oscillators (magnetrons) 50 are attached to different sides of the reflecting container 40, and microwaves are emitted toward the heating tube 20 inside the reflecting container.

The heating tube 20 is installed in the center of the spatial region 40M of the reflecting container 40 by a support member (not shown), and the heated object 30 is held beside it by a holding member (not shown). The heated object may be, for example, ceramics, semiconductors, or organic matter. Furthermore, the microwave heating device 10 includes a power supply circuit (not shown) capable of controlling the microwave oscillator 50, and a thermometer (radiation thermometer) (not shown) that measures the temperature of the heating tube 20, and the power supply circuit controls the microwave oscillator 50 based on the temperature of the heating tube 20 detected by the thermometer.

As shown in FIG. 2, the heating tube (hereinafter also referred to as the heating element) 20 is a container in which the heating material is placed, and here consists of a sealed, bottomed tubular container with an inlet tube 22 formed at one end, and is heat-formed from a quartz material that transmits microwaves. However, the container configuration is not limited to this, and it may be, for example, a disk-shaped container with an inner diameter large relative to its axial length. Carbon heating material 70 is filled inside the heating tube 20 so as to almost fill the entire inside of the heating tube 20.

Here, the carbon heating material 70 is composed of flowable carbon powder 70M. The carbon powder 70M is obtained by crushing a lump of carbon material, and the carbon powder 70M, which is an aggregate of particles having a specified particle size, is produced by so-called coarse crushing or medium crushing.

Since each particle of the carbon powder 70M has an irregular, uneven surface, gaps are created between the particles when the carbon powder 70M is placed inside the heating tube 20. The gaps between the particles are present throughout the entire carbon heating material 70. The inner diameter of the heating tube 20 and the filling rate of the carbon powder 70M are determined so that heat generation due to microwave absorption is dominant, as described below.

As shown in FIG. 2, a spatial region S filled with a rare gas is formed over the entire tube axial direction between the carbon heating material 70 and the vertical upper end surface of the heating tube inner surface 20U. However, since the size of the spatial region S is exaggerated in FIG. 2, the overall thickness of the carbon heating material 70 composed of carbon powder particles 70M is approximately equal to the inner diameter of the heating tube 20. In addition, the inside of the introduction tube 22 formed at the end of the heating tube 20 is not filled with carbon powder particles 70M, and a spatial region filled with a rare gas is formed.

The remaining space in the heating tube 20, excluding the carbon heating material 70, is filled with a rare gas. Here, one of the following gases is sealed: helium, neon, argon, or xenon, or two or more of these gases. The rare gas is inactive to the carbon heating material 70, and is also inactive to the heating tube 20 made of quartz glass.

When microwaves are applied to carbon powder 70M, which has gaps between the particles, and a heating tube 20 containing a rare gas sealed inside, the microwaves reach the entire carbon powder 70M. In particular, they reach the carbon powder near the center (axis) of the heating tube 20.

As a result, a high temperature state can be reached in a short time. In particular, by sealing the rare gas inside the heating tube 20, it is possible to prevent the carbon powder 70M from reacting with oxygen in the air to turn into carbon dioxide, which reduces the weight and reduces the filling amount.

When the carbon powder 70M generates heat, the heat generated in the carbon powder 70M near the center is released to the outside of the heating tube 20. As a result, the heat radiation of the heating tube 20 as a whole is increased, and the heated object can reach a high temperature in a short time.

The inner diameter of the heating tube 20 and the filling rate of the carbon powder 70M are set to increase the surface area of the carbon heating material 70 irradiated with microwaves, thereby realizing a rapid temperature rise. If the inner diameter of the heating tube 20 is too small, the amount of carbon powder 70M will be small, resulting in a reduced heat generation effect. On the other hand, if the inner diameter of the heating tube 20 (total thickness of the carbon heating material 70) is too large, the microwaves will not reach the center of the heating tube 20, and the temperature rise due to thermal conduction will become dominant, preventing the heating from reaching a high temperature in a short time. For example, the inner diameter of the heating tube 20 can be set to 1 mm to 100 mm.

If the filling rate of the carbon powder 70M is too small, the heat radiation will decrease due to the small filling amount, and the carbon powder 70M will be misaligned in the radial or axial direction inside the heating tube 20, resulting in an uneven high temperature state, and the load on the heating tube 20 will be large. On the other hand, if the filling rate is too large, it will be difficult for the microwaves to reach the center of the heating tube 20, making it difficult to reach a high temperature state in a short period of time. The filling rate is set here to a range of 0.05 to 0.2. However, the filling rate represents the volume ratio of the carbon powder 70M to the volume of the space inside the heating tube 20.

The heating tube 20 filled with rare gas and carbon heating material 70 can be manufactured as follows. A microwave-transparent quartz material is heated to create a bottomed cylindrical container with an introduction tube at one end, and carbon material (carbon powder) that absorbs microwaves and generates heat is filled into the container through the introduction tube. A rare gas that is inactive to the carbon material is then filled through the introduction tube to a specified range. For example, the range is set to 1 kPa to 40 kPa. The introduction tube is then heated and softened to seal the container, sealing it.

The pressure of the enclosed rare gas can be determined from the pressure resistance characteristics of the container and the relationship in which air pressure is proportional to the absolute temperature of the gas. For example, if a heating tube with a room temperature of 300 K is initially heated by microwaves to a high temperature of 2000 K, and the design conditions are such that the pressure inside the heating tube at that time is the same as atmospheric pressure, then the gas pressure in the heating tube at room temperature will be 15.195 kPa. Also, if a heating tube with a room temperature of 300 K is initially heated by microwaves to a medium temperature of 800 K, and the design conditions are such that the pressure inside the heating tube at that time is the same as atmospheric pressure, then the gas pressure in the heating tube at room temperature can be increased to 37.988 kPa.

The microwave heating device of this embodiment thus comprises an airtight quartz heating tube 20, a reflecting container 40 that houses the heating tube 20, and a microwave oscillator 50 that generates microwaves within the reflecting container 40. The inside of the heating tube 20 is filled with carbon heating material 70 made of carbon powder 70M, and rare gas is also enclosed.

This configuration allows the heating tube 20 to heat up in a short time, enabling the object to be heated 30 to be heated and baked at high speed. This also reduces power consumption compared to conventional electric furnaces. In addition, the heating tube 20, which serves as the heat source, can be easily replaced, simplifying maintenance.

The microwave heating device (microwave baking furnace) described above uses a rectangular reflecting container, but it may also be cylindrical or roughly spherical. In addition, the object to be heated is placed next to the heating tube 20, which is the heating element, in the reflecting container, but the object to be heated may also be placed surrounded by the heating tube 20. For example, it can be configured as a muffle furnace surrounded by multiple heating tubes 20. Alternatively, it can be used as a heat source for other heat treatment devices.

As for the configuration of the carbon heating material, it is also possible to use a lump carbon heating member instead; for example, it is possible to arrange a lump carbon heating member in which carbon plates are arranged in the axial direction. It is also possible to configure it so that gas (rare gas) that flows into the heating tube is sent and heated by the carbon heating material, and the heated gas is then released. Furthermore, it is also possible to place an object to be heated in the hollow part of a hollow cylindrical (double tube structure) heating tube and heat it.

The material of the heating tube may be other than quartz, as long as it transmits microwaves and does not react with gases such as rare gases. Furthermore, a fluid that is inert to microwaves and is a gas other than a rare gas may be used. The gas contained in the container may not be filled with a gas that is inert to the carbon heating material, but may be configured to contain some active gas.

Next, a microwave heating device according to a second embodiment will be described with reference to Figures 3 to 5. In the second embodiment, the temperature rise and fall of the heating element is controlled.

Figure 3 is a schematic block diagram of a microwave heating device according to a second embodiment.

As in the first embodiment, the microwave heating device 100 has a reflector container 40 that houses the material to be heated 30 and a heating tube (heating element) 20 in which carbon powder 70M is enclosed, and a microwave generator 150 irradiates microwaves to the heating element 20. The microwave generator 150 is driven by a power supply unit 86 and is connected to an AC transformer 88.

The thermometer (temperature measurement unit) 90, a radiation thermometer used here, measures the surface temperature of the heating element 20 at a predetermined time interval (e.g., several milliseconds). The thermometer controller 92 converts the measured temperature into a voltage and sends the voltage data to the controller 84. The controller (control unit) 84 controls the microwave output of the microwave oscillator 150 by sending a signal to the power supply unit 86 that controls the drive voltage of the microwave oscillator 150 based on the temperature-voltage conversion data. The heating element 20 is controlled to increase and decrease its temperature by this drive voltage control.

The operation unit 82 is operated by the operator when setting the temperature of the heating element, and an operation signal is sent to the controller 84. Here, it is possible to set a temperature of 800°C or higher.

Figure 4 is a graph showing the drive power of the microwave oscillator 150 with respect to microwave heating over time. Figure 5 is a graph showing the temperature of the heating element with respect to microwave heating over time. Note that in Figure 5, the temperature rises first due to the output of several millivolts before the microwave oscillator 150 turns on.

When the target temperature T0 (here, 1100°C) is set by the operator, the controller 84 in accordance with the target temperature T0 controls the power supply unit 86 to gradually increase the power to the microwave oscillator 150. As shown in FIG. 4, the rate of increase in power is approximately constant during a predetermined period V1 from the start of operation. During this predetermined period V1, the heating rate (°C/sec) of the heating element 20 is approximately constant, and the temperature of the heating element 20 increases at a heating rate of 30 (°C/sec) or more (see L1 in FIG. 5).

When the temperature of the heating element 20 reaches a temperature T1 (near-target temperature) close to the target temperature T0, the controller 84 reduces the power and performs voltage control by repeatedly increasing and decreasing the power (see period V2 in FIG. 4). As a result, the power at the near-target temperature T1 becomes the maximum power, and thereafter, power control is performed at a lower power than this.

While the power is being increased or decreased, the temperature of the heating element 20 rises at a slower rate than during period V1, and gradually approaches the target temperature T0 (see L2 in FIG. 5). When the temperature of the heating element 20 reaches the target temperature T0, the power is controlled so that the temperature of the heating element 20 is maintained at the target temperature T0.

When lowering the temperature of the heating element 20, the power is gradually reduced (see period V3 in FIG. 4). This causes the temperature of the heating element 20 to drop at a substantially constant rate of temperature drop (°C/sec). Here, the power is reduced so that the temperature drops at a rate of 30°C/sec or more. By keeping the rate of power reduction substantially constant, the rate of temperature drop also becomes substantially constant (see L3 in FIG. 5).

Figure 6 is a control block diagram for controlling the temperature of the heating element 20.

In the operation unit 82, the operator sets the target temperature (hereinafter, the target temperature is referred to as "TT"), the maintenance time TK of the target temperature TT, the temperature increase rate (temperature increase speed) TI, and the temperature decrease rate (temperature decrease speed) TD. The temperature T measured by the radiation thermometer is converted to a voltage TV in the thermometer controller 92 and sent to the controller 84.

The controller 84 is equipped with a temperature regulator 84A that executes PID control (not shown) to control the relationship between temperature and drive voltage. The temperature regulator 84A determines a power supply control voltage MV, which is a drive voltage control signal, based on the difference between a drive voltage (hereinafter referred to as a target drive voltage) for a temperature (hereinafter referred to as a target temperature) input to the controller 84 and a drive voltage (hereinafter referred to as a measured drive voltage) corresponding to the temperature of the heating element 20 measured at a predetermined time interval. The controller 84 sends the determined power supply control voltage MV to the power supply unit 86. The power supply unit 86 changes the drive voltage QV in microseconds under the control of the controller 84.

To be more specific about PID control, P (proportional constant) is used to change the output (drive voltage) according to the difference between the current temperature and the target temperature (the difference between the target drive voltage and the measured drive voltage). In addition, I (integral constant) is used together with P (proportional constant) to calculate the heat storage capacity and suppress overshoot and hunting. For sudden temperature changes due to disturbances, D (differential constant) is used to correct the drive voltage. The power supply unit 86 outputs power (W) according to the drive voltage QV to the microwave oscillator 150. Note that the temperature regulator 84A detects the optimal PID constant by prior auto-tuning in order to perform optimal temperature control.

During the time period when the heating element 20 is being heated, the temperature regulator 84A performs PID control based on the target temperature TT and the temperature rise rate TI. The temperature regulator 84A determines the power supply control voltage MV for the power supply unit 86, and the controller 84 changes the drive voltage QV (power) for the microwave oscillator 150. P (proportional constant) is determined according to the set temperature rise rate TI.

During the time period in which the increased temperature of the heating element 20 is to be maintained, the temperature regulator 84A performs PID control based on the target temperature TT and the temperature maintenance time TK. During the time period in which the temperature of the heating element 20 is to be decreased, the temperature regulator 84A performs PID control based on the temperature decrease rate TD.

In this manner, according to this embodiment, precise temperature control can be achieved during the heating and cooling processes of the heating element 20 by controlling the power of the microwave oscillator 150. As a result, precise heating of the object to be heated can also be achieved.

The rate of temperature rise of the heating element depends on the mass of the carbon powder 70M, and when the mass is large, precise temperature control is difficult using proportional control alone. On the other hand, the carbon powder 70M dissipates heat quickly. In this embodiment, PID control is performed by the temperature regulator 84A according to the characteristics of the carbon powder 70M, which is the object to be heated. This makes it possible to suppress overshooting and hunting and to increase the temperature of the heating element 20 asymptotically near the target temperature, so that despite a sudden temperature rise, the target temperature can be reached efficiently without disturbances such as unintended temperature fluctuations up and down.

In addition, by gradually increasing the power of the microwave oscillator 150 after the start of operation, the power reaches maximum after a certain period of time when microwave absorption has stabilized, reducing the generation of reflected waves and preventing damage to the microwave oscillator 150. Conversely, by gradually decreasing the power of the microwave oscillator 150, the temperature of the heating element 20 can be reduced at a rate of 30 (°C/sec) or more.

Below, we will explain an example that corresponds to the second embodiment.

The example is a microwave heating device corresponding to the second embodiment, in which a heating tube with an outer diameter of 6 mm, an inner diameter of 4 mm, and a tube length of 60 mm was filled with carbon powder at a filling rate of 0.08, and rare gas was sealed in at 1.4 kPa. Then, a heating experiment was performed by irradiating a 2.45 GHz microwave in a spherical reflecting container and setting the target temperature to 1100°C. In the heating experiment, temperature control was performed by a controller as in the embodiment, and a radiation thermometer with a measurement wavelength of 900 nm was used to measure the surface temperature of the heating tube.

As a result of the experiment, it was possible to confirm temperature changes that drew a curve similar to that shown in Figure 5, and it was found that the heating tube 20 was heating at a heating rate of 30 (°C/s) or more. It was also confirmed that the temperature rose asymptotically near the target temperature, and that the target temperature was maintained as shown in Figure 5 even after the target temperature was reached.

20 Heating tube (heating element)
30 object to be heated 40 reflector container 70 carbon heating material (heating material)
70M Carbon powder 84 Controller 90 Thermometer 100 Microwave heating device 150 Microwave generator (magnetron)

Claims (10)

A heating device that irradiates microwaves to ceramics, semiconductors, or organic materials other than food as objects to be heated,
A reflecting container that reflects microwaves;
A heating element including a powder containing carbon that absorbs microwaves and generates heat, and a container that transmits microwaves and is filled with the heating element, the heating element being installed in the reflecting container;
A temperature measuring unit for measuring a temperature of the heating element,
The object to be heated is placed in the reflecting container at a distance from the container,
The heat generated by the heat generating material is transferred to the object to be heated through the spatial region of the reflector container,
A heating device characterized in that the temperature of the heating element is increased to a temperature near a target temperature at a heating rate of 30 (°C/sec) or more, and when the temperature near the target temperature is reached, the heating element is increased to asymptotically approach the target temperature, and when the target temperature is reached, the temperature of the heating element is maintained at the target temperature by continuously irradiating the heating element with microwaves from the start of irradiation based on the measured temperature. The heating device according to claim 1, characterized in that the heating element is cooled at a rate of 30°C/sec or more. A magnetron that irradiates microwaves to the heating element;
A control unit for controlling the power of the magnetron,
3. The heating device according to claim 1, wherein the control unit controls the power of the magnetron to raise the temperature of the heating element at a heating rate of 30 (.degree. C./sec) or more. The heating device according to claim 3, characterized in that the control unit gradually increases the power of the magnetron from the start of operation until the target temperature is reached. The heating device according to claim 4, characterized in that the control unit reduces the power when the heating element reaches a temperature close to the target temperature, and repeats increasing and decreasing the power multiple times. The heating device according to claim 2, characterized in that the control unit gradually reduces the power when lowering the temperature of the heating element at a temperature drop rate of 30 (°C/sec) or more. The power to the magnetron is 100 W or more;
7. The heating device according to claim 3, wherein the target temperature is 800[deg.] C. or higher. The heat generating material is made of flowable carbon powder,
8. The heating device according to claim 1, wherein the packing rate of the powder or granular material is set in the range of 0.05 to 0.2. The heating device according to any one of claims 1 to 8, characterized in that a fluid inert to the heat generating material is sealed in the container. A heating method for irradiating microwaves to a heating element that is placed in a reflecting container that reflects microwaves, the heating element being made of a powder containing carbon and absorbing microwaves to generate heat, the heating element being made of a powder containing carbon and absorbing microwaves to generate heat, the heating element being placed in a reflecting container that reflects microwaves , the method comprising the steps of:
The object to be heated is placed away from the container in the reflecting container, and heat generated by the heat generating material is transferred to the object to be heated through a spatial region of the reflecting container;
Measure the temperature of the heating element;
A heating method characterized in that the temperature of the heating element is increased to a temperature near a target temperature at a heating rate of 30 (°C/sec) or more, and when the temperature near the target temperature is reached, the heating element is increased to asymptotically approach the target temperature, and when the target temperature is reached, the temperature of the heating element is maintained at the target temperature by continuously irradiating the heating element with microwaves from the start of irradiation based on the measured temperature.

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