JPH03184296A - Dielectric heating - Google Patents

Abstract

PURPOSE:To obtain a dielectric heating method permitting the heating melting of a high melting-point dielectrics by applying dielectric heating in the low temperature region of a heated substance, and induction heating in the high temperature region. CONSTITUTION:A microwave generation device 1 generating a microwave of 3GHz extent, reflection wave prevention isolator 2, power monitor 3, impedance cooker 4, and discharge detector 5 are connected in a column with a waveguide to compose an dielectric heating system line 20. Moreover a high-frequency generation device 6 generating a high frequency of 400K-5MHz extent, matching observing device 7, and matching cooking device 8 are also connected in a column to compose an induction heating system line 30. These two series of the heating system lines 20 and 30 are selectively applied to an applicator 10 as a high-frequency melting furnace by the switching operation of a switching device 9. This enables a heated substance 11 to be effectively heated to high temperature to be melted.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a method for heating dielectric materials, and in particular, to heating high melting point dielectric materials such as new ceramics, glass materials, and semiconductor materials at high temperatures by combining dielectric heating and induction heating. This invention relates to a method for heating a dielectric material that can be melted.

[Background of the invention]

Heating of dielectric materials using microwaves can be performed in a short time and with high efficiency due to the extremely high electric field strength generated in a tuned applicator (high frequency melting furnace). However, the heating temperature that can be reached is limited due to the following reasons.

■ In dielectric heating of dielectric materials, microwave penetration depth L (
Depth where the electric field strength becomes 1/e, e...base of natural logarithm>
is given by the following equation and is proportional to the wavelength λ.

ε is the relative dielectric constant of the dielectric, and tan δ is the dielectric loss angle rate.

On the other hand, the amount of heat generated Q is: Q= (AE”/λ)XtrXtanδ ・(2)
The shorter the wavelength λ, the better the efficiency, where A is the proportionality constant and E is the electric field strength.

In other words, L and Q have a contradictory relationship when the wavelength λ is used as a variable. Therefore, in order to obtain optimal heating, it was necessary to select the wavelength λ taking both factors into consideration.

However, in most dielectric materials, as the temperature rises, the mobility of internal ions increases and the conductivity σ tends to increase.

Further, the values of ε and tan δ generally increase as the temperature increases, and as a result, the depth L into which the microwave penetrates decreases.

L in the above equation (11) is σ-0, that is, in the case of a dielectric material, but as this σ increases, the current due to the microwave electric field will only flow within the depth range of the so-called skin depth δ.This δ is given by: Thus, when λ is small and σ is large, δ becomes small. In this case, only the surface is heated. C is the speed of light, and μ is the magnetic permeability.

■ Also, when the heated object becomes high temperature, it generates radiation given by the following blank radiation formula. That is, the wavelengths are λ and λ+
The radiant energy density ρdλ between dλ is given by the following formula.

...(4) Here, h is a blank constant, k is a Portzmann constant, and T is an absolute temperature.

Here, when T exceeds 1500° C. in terms of Celsius, the object to be heated starts to radiate light with a wavelength in the ultraviolet region. At this time, the short-wavelength ultraviolet rays ionize the gas and generate ions, so that a discharge occurs due to the microwave electric field being irradiated at that time.

Therefore, microwave power is consumed by this discharge, and further temperature rise of the object to be heated is suppressed.

[Problem to be solved by the invention]

However, in reality, it is desired in various fields to melt objects to be heated that have high melting points. If this is done using the conventional microwave dielectric heating method, the heating progresses and the temperature rises as described above, and due to the increase in conductivity and the occurrence of discharge, the temperature does not rise above a certain point. Not only has the limit been reached and the above requirements cannot be met, but there are also problems such as an increase in power consumption.

An object of the present invention is to provide a dielectric heating method that makes it possible to heat and melt a high melting point dielectric by applying a dielectric heating method to a low temperature range and an induction heating method to a high temperature range.

[Means to solve the problem]

For this purpose, the present invention dielectrically heats a heated object made of a dielectric material by microwave irradiation, and when the electrical conductivity of the heated object reaches a value that allows it to be regarded as a conductor, and/or when the heated object When discharge occurs, the microwave irradiation is stopped or cut off, and the object to be heated is induction heated by applying a high frequency magnetic field.

〔Example〕

Hereinafter, a heating method according to an embodiment of the present invention will be explained. FIG. 1 is a block diagram of a heating bag ff1A for carrying out the heating method of one embodiment. 1 is a microwave generator that generates microwaves of about 3 Gllz, 2 is an isolator for preventing reflected waves, 3 is a power monitor, 4 is an impedance adjuster, and 5 is a discharge detector, which are connected in series with a waveguide. The dielectric heating system line 20 is connected to the dielectric heating system line 20.

Further, 6 is a high frequency generator that generates a high frequency of about 400 to 5 MHz, 7 is a matching monitoring device, and 8 is a matching adjustment device, which are also connected in series to form an induction heating system line 3o.

These two heating system lines 2o and 3° are selectively applied to the applicator 10 as a high frequency melting furnace by switching operation of the switching device 9. In other words, the heating device A of this embodiment can be set to either the dielectric heating mode or the induction heating mode by a switching operation. Note that 11 is an object to be heated housed in the applicator 10.

The isolator 2 is for preventing the microwave reflected wave from the applicator 1o from returning to the generator 1.

The power monitor 3 is a device that monitors the matching of the heated object 11 placed in the applicator 10,
The power input to the applicator 10 and the power reflected from the applicator 10 are detected and displayed. If the above matching is not achieved, the reflected power will increase.

The impedance adjuster 4 is a device for adjusting the power reflected from the applicator 10 to minimize it.

The discharge detector 5 is a detector for detecting discharge of the object 11 to be heated.

The matching monitoring device 7 and the matching adjusting device 8 are devices for performing matching adjustment according to the properties (conductivity, etc.) of the object to be heated 11, and these are the power monitor 3 and impedance adjuster 4 of the dielectric heating system line 20. and each operates in the same way.

Now, the applicator 10 is heated by the heating device A of this embodiment.
To heat the object to be heated 11 inside, first, the switching device 9 is operated to select the dielectric heating mode and the microwave generator 1 is activated.

As a result, the microwave generated in the microwave generating bag N1 enters the applicator 10 through the reflected wave prevention isolator 2, the power monitor 3, the impedance adjuster 4, the discharge detector 5, and the switching device 9, and enters the heated object 11. irradiation to perform dielectric heating.

At this time, if the reflected power from the applicator 10 is displayed large on the power monitor 3 and it is reported that the matching is mismatched, the impedance adjuster 4 is adjusted so that the reflected power is minimized while watching the power monitor 3. Adjust. This adjustment allows efficient dielectric heating.

When the heated object 11 rises in temperature and its electrical conductivity increases (to the extent that it can be regarded as a conductor), and becomes heated only by surface current, matching becomes extremely difficult. At the same time, as the temperature of the object to be heated 11 increases, ultraviolet rays are emitted and discharge tends to occur, which is detected by the discharge detector 5.

When this state is reached, the dielectric heating system line 20
is stopped, the heating device A is switched from the dielectric heating mode to the induction heating mode by the switching device 9, and the operation of the induction heating system line 30 is started.

That is, by operating the high frequency generator 6, the object to be heated 11
A high frequency magnetic field is applied to the heated object 11 to directly generate an induced current in the heated object 11 . At this time as well, the matching is adjusted by the matching adjustment device 8 while watching the matching monitoring word W7. As a result, the object to be heated 11 is heated and melted by induction heating above a temperature that could not be raised in the dielectric heating mode.

FIG. 2 is a block diagram of a heating device B according to another embodiment.

In this example, two types of applicators 10, 10 are used: an applicator 10 for dielectric heating and an applicator 10' for induction heating.
``, and each is applied independently to the dielectric heating system line 20 and the induction heating system line 30.In addition, a switching device 9'' is connected to the power source 12, and the microwave generator 1 and the high frequency generator 6 to enable switching.

That is, in this example, the heating device B can be selectively set to either the dielectric heating mode or the induction heating mode by switching the switching device 9'. In this case, the object to be heated 11 is heated by the applicator 10.10 in accordance with the switching of the heating mode.
It is necessary to exchange and move between '.

FIG. 3(a) fb) (C1 is a diagram showing a specific example of the heating system switching method in each of the above embodiments.

FIG. 3<a) shows a switching method applicable to the heating bag WA of the embodiment shown in FIG. 1, in which the heating mode within the applicator 10 can be changed by moving the coil 13 in and out of the applicator 10.

That is, in the case of the dielectric heating mode, the coil 13 is taken out of the applicator 10 and microwaves are applied from the dielectric heating system line 20. Next, in the case of induction heating mode, a coil 13 is fitted to the outer circumference of an alumina crucible 14 for storing the object to be heated 11 installed in the applicator 10, and a coil 13 is connected to the induction heating system line 30. Supply high frequency current.

FIG. 3 (bl (C)) shows a switching method that can be applied to the heating device B of the embodiment shown in FIG.
is installed in the applicator 10 in the lower side adjacent room of the applicator 10.1. The crucible 14 is mounted on a vertically movable pedestal 15 and moved between the applicator 10 and the coil 13, so that both dielectric heating mode and induction heating mode can be set.

In the case of the dielectric heating mode, the crucible 14 is pushed up and held within the applicator 10 by the pedestal 15, and the object to be heated in the crucible 14 is dielectrically heated by the supplied microwaves. At this time, the pedestal 15 is fitted into the crucible through hole 10b formed in the partition wall 10a of the applicator 10, 10' to prevent leakage of microwaves from the applicator 10.

In the case of induction heating mode, the crucible 14 is pulled down into the applicator 10' by the pedestal 15 and fitted inside the coil 13, and the object to be heated in the crucible 14 receives a high frequency magnetic field from the coil 13 and is heated by induction. .

In addition, (in the ClO example, for example, a crucible 14 around which a coil 13 is wound is provided below the applicator 10, and the applicator 1
The object to be heated 11 dielectrically heated at zero flows into the crucible 14 and is heated by induction by the coil 13.

In the above embodiments, switching from dielectric heating to induction heating is performed by detecting the discharge generated in the object to be heated, but it is of course possible to switch by detecting the electrical conductivity of the object to be heated. . In this case, for example, conductivity (resistance) detection timing may be set a plurality of times, and microwave irradiation may be interrupted only at this time.

〔Effect of the invention〕

From the above, according to the present invention, dielectric heating is applied to the object to be heated in the low temperature range, and induction heating is applied to the object to be heated in the high temperature range. Even if the temperature of an object becomes high and its conductivity increases or electric discharge occurs and it becomes difficult to raise the temperature further, you can switch to induction heating to efficiently heat the object to a higher temperature. It has the advantage that it can be melted.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a heating device for carrying out the dielectric heating method of the present invention, FIG. 2 is a block diagram of a heating device of another embodiment, and FIG. It is an explanatory diagram showing a switching method between a heating method and an induction heating method. 1... Microwave generator, 2... Isolator for preventing reflected waves, 3... Power monitor, 4... Impedance adjuster, 5...Discharge detector, 6...High frequency generator, 7...Matching monitoring device, 8...Matching adjustment device, 9.9'...Switching device, 10.10°
... Applicator, 11... Heated object, 12...
Power supply, 13... Coil, 14... Crucible, 15...
Pedestal, 20... Dielectric heating system line, 30... Induction heating system line, A, B... Heating device.

Claims (1)

[Claims] (1) When a heated object made of a dielectric material is dielectrically heated by microwave irradiation, and the electrical conductivity of the heated object reaches a value that allows it to be regarded as a conductor, and/or a discharge occurs in the heated object. When this happens, the microwave irradiation is stopped or cut off, and the object to be heated is heated by induction by applying a high frequency magnetic field.