JP3233300B2 - Microwave sintering method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[Industrial application field] Dielectric loss factor (= ε _r · tan δ: ε _r
Small material dielectric constant, tan [delta has temperature dependency is large thermal conductivity of the dielectric loss tangent), e.g., by microwave titanium dispersed lead zirconate titanate (PZT) ceramics, evenly be heated, be uniformly sintered Possible microwave
Sintering method .

[0002]

2. Description of the Related Art When a material having a large temperature dependence of a dielectric loss factor and a small thermal conductivity, such as PZT ceramics, is heated by a microwave, a temperature distribution is extremely uneven due to a difference in a dielectric loss factor in a sintered body. , A so-called runaway phenomenon occurs, and uneven heating occurs in the sintered body, making it difficult to perform uniform heating or sintering. As a method for solving the above-mentioned problems, the present inventors have proposed a method in which the temperature dependency of the dielectric loss factor is smaller than that of a sintered body, and a microwave absorber such as SiC having a property of having a large thermal conductivity. Irradiation of microwaves on a sintered body such as PZT can prevent a rapid change in the dielectric loss factor of the sintered body, and it has been confirmed that uneven heating can be prevented (Japanese Patent Laid- Open No. 4-92870).
issue). This method can prevent uneven heating because the microwave absorber absorbs a part of the microwave energy, so that the sudden change in the dielectric loss factor of the sintered body is reduced, and the microwave absorber is used. Due to the heat conduction from the contact portion between the sinter and the sinter, and the radiant heat from the microwave absorber to the sinter, the non-uniform temperature of the sinter is eliminated. Can be uniformly heated.

However, when the temperature becomes higher than 1100 ° C.,
Uneven heating may occur. In particular, when a relatively small frequency of 2.45 GHz was used, uneven heating became large. At a high temperature of 1100 ° C. or higher, depending on the material of the microwave absorber and the material to be sintered, the microwave absorber and the material to be sintered react with each other, and a compound layer having a large dielectric loss factor is formed on an interface between them. Formed, a sharp temperature distribution occurs in the sintered body, causing uneven heating of the sintered body, or a reaction between the microwave absorber and the sintered body to cause the microwave absorber to be formed in the sintered body. There is a problem that a part of the element diffuses, the sintered body has an uneven structure, and the characteristics are deteriorated.

[0004]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method of sintering ceramics having a large temperature dependence of a dielectric loss factor and a small thermal conductivity by microwaves.
At the above-mentioned high temperature, uneven heating is surely eliminated, and sintering can be performed uniformly.

[0005]

[Means for Solving the Problems] (Structure of the First Invention) A microwave sintering method according to the first aspect of the present invention (the invention according to claim 1) includes a microwave generating means for generating a microwave, and a sintering method. A cavity resonator for heating and sintering the body,
A waveguide for transmitting microwaves from the microwave generation means to the cavity resonator, and a coupler for introducing microwaves from the waveguide to the cavity resonator to maintain resonance in the cavity resonator consists of a, using a microwave heating apparatus for sintering the object sintered body is irradiated with microwaves in the sintered body arranged in the air cylinder resonator, large dielectric loss factor is, the microwave And a microwave absorber having a smaller temperature dependency of a dielectric loss factor than the sintered body and the sintered body directly.
Or placed in the cavity with indirect contact
The low-loss body having a low dielectric loss factor and easy transmission of microwaves is disposed on the sintered body supporting means.
To cover the body and part of the microwave absorber in a non-contact manner
And irradiate the object with microwaves to remove the object.
It is characterized by sintering .

[0006] (Configuration of Second Invention) sintering the method according to the microwave of the present second invention (invention described in claim 2), in the sintering method <br/> microwave of the first invention, the object to be between the sintered body and the microwave absorber, the object to be sintered body and reacting with the microwave absorber difficulty
Ku, and the dielectric loss factor is placed a small insulator,該被sintered body
And the microwave absorber in an indirect contact state.
Cormorant be characterized. (Structure of the third invention) The microwave sintering method of the third invention (Claim 3)
Invention) is the microwave sintering method of the first invention.
In the above, the object to be sintered and the microwave absorber are both
Heated and sintered in the form of a plate and stacked
And features.

[0007]

(Function of the first invention) In the microwave sintering method of the first invention, at least a part of the object to be sintered and the microwave absorber are covered with a low-loss material so that they can be heated at a high temperature of 1100 ° C or more. It is presumed that the reason why uneven heating of the sintered body can be prevented is as follows. That is, when the sintered body and the microwave absorber are not covered with the low-loss body, the electric field intensity is maximum at the position of the sintered body. However, it is considered that, by covering the sintered body or the like with the low-loss body, the electric field intensity at the position of the sintered body, which is the center of the low-loss body, is reduced and becomes almost uniform. Therefore, it can be considered that the electric field can be prevented from being concentrated on the sintered body, the electric field intensity inside the sintered body is substantially uniform, and the temperature distribution is hardly uneven, and uneven heating can be prevented. Further, by covering with a low-loss body, a discharge generated in the sintered body and the inner wall of the cavity can be prevented, and stable heating can be performed.

When a low-loss body is not used, the lower the frequency, the deeper the penetration of microwaves. Therefore, the effect of preventing the above-described uneven heating of silicon carbide (SiC) as a microwave absorber is obtained. 1. smaller than, for example, 6 GHz;
45 GHz tends to cause uneven heating at high temperatures even when SiC is used. However, since the effect of covering the sintered body with the low-loss body is independent of the frequency, the effect of equalizing the electric field intensity at the center of the low-loss body by the low-loss body can be used even at a low frequency of 2.45 GHz. No uneven heating at high temperature.

(Function of the Second Invention) In the microwave sintering method according to the second invention, it is difficult for the microwave sintering to react between the sintered body and the microwave absorber between the sintered body and the microwave absorber. When an insulator is provided, the reaction between the sintered body and the microwave absorber can be prevented while preventing uneven heating of the sintered body. The reason is presumed to be as follows. Since the insulator is installed between the sintered body and the microwave absorber, the sintered body and the microwave absorber are not in contact with each other, preventing the reaction between the sintered body and the microwave absorber. it can. Further, since a material that does not easily react with the sintered body and the microwave absorber is used as the insulator, no reaction occurs between the sintered body and the microwave absorber and the insulator. Further, since the insulator has a small dielectric loss factor, the insulator itself is not heated, and the insulator hardly has a temperature distribution. Therefore, there is almost no thermal effect on the sintered body from the insulator, and the microwave absorber and the low-loss body can prevent uneven heating. (Operation of the Third Invention) In the microwave sintering method of the third invention,
The binder and the microwave absorber are both plate-shaped and
It is arranged in the state where it was placed. Sintered body and microwave absorption
Since the body is plate-shaped and stacked on top of each other,
Large contact area and high integration. This makes it more effective
Reduces sudden change in dielectric loss factor of sintered body and absorbs microwave
The temperature of the sintered body is made uniform by the heat conduction of the body,
Can be prevented.

[0010]

(Effect of the First Invention) According to the microwave sintering method of the first invention, the PZ
Even if the material has a large temperature dependence of the dielectric loss factor such as T and has a low thermal conductivity, even when a relatively high frequency of 2.45 GHz is used at a high temperature of 1100 ° C. or more, the heating unevenness is surely prevented. Can be eliminated,
It can be uniformly sintered.

(Effect of the Second Invention) According to the microwave sintering method of the second invention, the reaction between the sintered body and the microwave absorber is prevented while preventing uneven heating of the sintered body. I can . (Effect of Third Invention) According to the microwave sintering method of the third invention,
Depending on the shape and arrangement of the binder and microwave absorber,
Effectively eliminates uneven heating of the sintered body and sinters it uniformly.
Can be

[0012]

Embodiment (Specific Example of First Invention) The microwave sintering method of this specific example is used in the present invention.
Microwave generating means for generating microwaves, a cavity resonator for heating and sintering the object to be sintered, a waveguide for transmitting microwaves from the microwave generating means to the cavity resonator, and the waveguide A coupler that introduces microwaves from a tube into the cavity resonator and maintains resonance in the cavity resonator is typically a microwave generator that generates microwaves and is used to heat and sinter a body to be sintered. There is no particular limitation as long as the configuration is used in a wave heating device. Microwave sintering of this example
FIG. 1 shows an example of a typical microwave heating apparatus used in the method . This apparatus comprises a microwave generating means 1 for generating microwaves, a cavity resonator 2 for heating and sintering a body to be sintered, and transmitting microwaves from the microwave generating means 1 to the cavity resonator 2. And a coupler 4 for introducing microwaves from the waveguide 3 into the cavity resonator 2 to maintain resonance in the cavity resonator, and a coupler 4 disposed in the cavity resonator.
A support rod 5 made of quartz for supporting the sintered body 6 and a motor 10 provided at the lower end of the support rod 5 for rotating the support rod. The microwave absorber 7 is disposed on the sintered body so as to cover the upper and lower surfaces thereof . Tube made of quartz, etc.
Low-loss body 8 is formed of the sintered body and the microwave sintered body.
Placed in the cavity resonator so as to cover a part thereof,
The object to be sintered is heated using a microwave heating device. The container 8 has a gas inlet 12 for introducing gas such as oxygen and a gas outlet 13 for discharging gas.

The object to be sintered, which can be sintered by the present sintering method , is such that when it is sintered by ordinary microwave heating, uneven heating tends to occur, that is, the temperature dependence of the dielectric loss factor is large, and the thermal conductivity is high. Is a material having a small value, specifically, ceramics having a temperature dependence of a dielectric loss factor of one digit or more in a temperature range from room temperature to 800 ° C. and / or a thermal conductivity of 0.1 cal / c.
It refers to ceramics with m · sec · ° C or higher. As such a material, for example, PZT, barium titanate (Ba)
There are ferroelectric materials such as TiO ₃ ) and PLZT.

[0014] Then, the microwave absorber, good microwave absorption greater dielectric loss factor is, the temperature dependence of the dielectric loss factor is small is used material from the sintered body. First, in order to sufficiently absorb microwaves, the dielectric loss factor at room temperature in a microwave band (0.3 to 30 GHz) must be larger than 0.1, depending on the type of the sintered body. desirable. When the dielectric loss factor is in the above range, the microwave absorber is mainly heated in a low temperature range of about 800 ° C. or less before the runaway phenomenon occurs in the sintered body, and the temperature distribution of the sintered body becomes more uniform. Further, it is desirable that the temperature dependency of the dielectric loss factor is smaller than that of the sintered body. If the temperature dependency is larger than that of the sintered body, a runaway phenomenon occurs in the microwave absorber and uneven heating of the sintered body is reduced. There is a risk of contributing.

The microwave absorber has a thermal conductivity of
Desirably, it is 0.1 cal / cm · sec · ° C. or more. If the thermal conductivity is smaller than this, a temperature distribution is generated in the microwave absorber, and the generated heat may be conducted to the sintered body to promote non-uniformity of the temperature distribution of the sintered body.
Such materials include SiC, ZnO, C (carbon),
AlN, B ₄ C and the like.

Further, the microwave absorber is a material to be sintered.
When it is placed directly or indirectly in contact state, as a result, it may be arranged so as to be prevented from uneven heating of the sintered body. Although the shape of the microwave absorber is not limited, it is usually used in the form of a plate, and its thickness is 0.5 to 10
mm is desirable. If it is less than 0.5 mm, the microwave absorption efficiency becomes small. On the other hand, if it exceeds 10 mm, most of the microwaves are absorbed by the microwave absorber, so that the energy efficiency is deteriorated and the electric field of the cavity resonator is disturbed, so that it is difficult to obtain resonance. Further, the microwave absorber,
It is desirable that the size of the object to be sintered be equal to or larger than the size after sintering. If it is small, a temperature distribution occurs in a portion not covered by the microwave absorber, and uneven heating may occur in the sintered body, which is not preferable.

Next, the low loss material has a dielectric loss factor is very small, using a material easily passes through the microwaves. The dielectric loss factor is preferably 0.01 or less at room temperature, and if it is larger than this, the low-loss body itself may be heated by absorbing the microwave, and the low-loss body is disposed in the cavity resonator. Sometimes the electric field in the cavity resonator is disturbed, making it difficult to obtain resonance, and the electric field strength in the low-loss body becomes uneven,
Uneven heating occurs in the sintered body. Further, it is desired that the temperature dependence of the dielectric loss factor of the low-loss body is extremely small. A large temperature dependency of dielectric loss factor, when the sintered body is heated, the temperature of the low-loss material is increased by radiant heat, the microwave is irradiated in this heated state, low loss material It is not preferable because uneven heating may occur.

As a material of such a low loss body, Si is used.
O ₂ , sapphire, or high-purity alumina with a purity of 99% or more is used. The low-loss body is arranged so as to cover at least a part of the microwave absorber and the sintered body in a non-contact manner. There is no particular limitation on how to cover, and it is only necessary to arrange so that the electric field intensity inside the area covered with the low-loss body becomes uniform. There is no particular limitation on the shape, for example,
A tubular body or a hollow body with a square or elliptical cross section is desirable,
Usually, a tube is used, and the object to be sintered is arranged at the center thereof.

[0019] The insulator used in the sintering process by microwaves of this example (example of the second invention), that resistivity at room temperature has a higher 10 ¹⁴ Ω · cm, a so-called insulator, In addition, the material must not easily react with the sintered body and the microwave absorber.
Further, it is desirable that the dielectric loss factor is 0.05 or less at room temperature. If the dielectric loss factor is larger than this, the insulator is also heated and a temperature distribution is generated in the insulator, and the sintered body is heated unevenly due to heat conduction, which may cause uneven heating of the sintered body. There is. When the material to be sintered is an oxide-based ceramic as such a material, Al ₂ O ₃ , MgO, Y ₂ O
_3, also, if the sintered body of non-oxide ceramics is preferably used BN and the like. Although the shape is not particularly limited, it is generally preferable to use a plate. The thickness of this insulator is desirably 5 mm or less. If it is thicker than 5mm,
It is not preferable because the distance between the microwave absorber and the sintered body is too large, and the effect of the microwave absorber is reduced. Further, the insulator is disposed in contact with the object to be sintered and the microwave absorber. (Embodiment of the Third Invention) In the microwave sintering method of this embodiment, the object to be sintered is
Both the microwave absorber and the microwave absorber have a plate shape. Further
The two are arranged in a stacked state.
is there. More effective by forming into a plate shape and laminating
Heating and sintering becomes possible.

(Example 1) PZT green compact as sintered body, microwave absorber
SiC plate, SiO as low loss body_TwoTubeMicrowave heating
Placed on the deviceIrradiated microwave, ceramic sintered body
Was manufactured, and the characteristics of the sintered body were evaluated. In this embodiment,
FIG. 1 schematically shows a microwave heating apparatus used in the method.
This device is a microwave generator that generates microwaves.
Step 1, rectangular parallelepiped cavity for heating and sintering the object to be sintered
Resonator 2, cavity resonator 2 from microwave generating means 1
Waveguide 3 for transmitting microwaves to and from waveguide 3
Microwaves are introduced into the cavity resonator 2 so that the cavity
Window to maintain resonance4 and, The cavity resonatorWithinArranged
A support rod 5 made of quartz for supporting the sintered body 6
A mode is provided at the lower end of the rod 5 for rotating the support rod.
Data 10. The sintered bodyAnd the microwave absorber
And the object to be sintered isTop and bottomTheMicrowave absorption
SiC as bodyIt is in a state of being laminated with. Also,From quartz
The tubular low loss body 8 is formed by
Placed in the cavity so as to cover a part of the body,
The object to be sintered is sintered using the microwave heating device. What
OhA gas for introducing oxygen gas into the low-loss body 8
An inlet and a gas outlet for discharging gas are installed.
To.

First, PZT powder, which is a sintered body, is
After the treatment, a PZT green compact sample having a diameter of 20 mm and a thickness of 3 mm was obtained. An SiC plate having a diameter of 20 mm and a thickness of 3 mm was used for the microwave absorber. This SiC has a dielectric loss factor of 1.3 and a thermal conductivity of 0.3 cal / cm · sec · ° C. Further, the low-loss body is made of SiO ₂ having an inner diameter of 25 mm and a thickness of 2 mm.
Tubes were used. The dielectric loss factor of this SiO ₂ is 0.001 or less.

Next, the PZT sample is sandwiched between SiC plates,
This was arranged at the maximum portion of the electric field in the cavity resonator by the support rod 5 made of quartz. Further, a low-loss SiO ₂ tube was inserted from the sample insertion hole 9 provided on the cavity wall surface to cover the sample and the SiC. The cavity resonator, with a single cavity resonator, 2.45 GHz resonant frequency, the electromagnetic field mode in the cavity resonator and TE _103, the sintering atmosphere is flowing oxygen (2l / min) in And The object to be sintered was rotated by the electric motor 10 via a support rod so that the electric field in the object to be sintered was uniform.

Next, the object to be sintered was heated and sintered.
By the present sintering method , 1200
Can be heated up to 1 ° C. or higher, 1150 ° C., 1 min
Under the sintering conditions, a sintered body having a density of 8.1 g / cm ³ and a true density was obtained. In addition, as a result of observation of the fracture surface, it was found that the sintered body was homogeneous.

For comparison, a sintered body similar to that of Example 1 was used except that a low-loss body made of a SiO ₂ tube was not used.
Heat treatment was performed using a microwave absorber and a microwave heating device. As a result, the central portion of the sample was melted by microwave heating to 1150 ° C., resulting in a non-uniform structure, and could not be sintered to near the true density. This is presumed to be because the sample had a temperature distribution due to no use of the low-loss SiO ₂ tube, resulting in uneven heating, and the center of the PZT sample was locally heated.

If a low-loss body is not used, discharge may occur in a part of the sintered body (PZT sample) or a part of the microwave absorber (SiC plate). It became stable and it was difficult to continue sintering any more. However, by covering the body to be sintered and the microwave absorber with a low-loss body, discharge was reliably prevented, and stable sintering was performed.

(Example 2) A PZT compact as a sintered body, a SiC plate as a microwave absorber, and a SiO ₂ tube as a low-loss body were used, each having the same size and material as in Example 1. An insulator is disposed between the sintered body and the microwave absorber, and the same microwave heating apparatus as in Example 1 is used to heat the same under the same microwave heating conditions as in Example 1 to influence the sintering on the insulator. The effects were investigated.

As shown in FIG. 2, PZT subjected to CIP processing
An Al ₂ O ₃ plate as an insulator having a diameter of 20 mm and a thickness of 1 mm was arranged on the upper and lower surfaces of the green compact sample, and a SiC plate as a microwave absorber was arranged on both Al ₂ O ₃ plates. The entire laminated body composed of the PZT sample and the like configured as described above is placed on a sample stage at the tip of a support rod, and these are placed in a low-loss S
It was covered with an iO ₂ tube and inserted into the cavity at the site of maximum electric field. The dielectric loss factor of Al ₂ O ₃ used for the insulator is 0.01.

Next, the object to be sintered was heated and sintered.
By the present sintering method , 1200
Can be heated to more than ℃, 1150 ℃, 5min
A sintered body having a density of 8.1 g / cm ³ and a true density was obtained. Further, as a result of observation of the fracture surface, it was revealed that the sintered body was homogeneous. In addition, element analysis in the thickness direction of the sintered body by EPMA analysis revealed that there was no diffusion of elements from the insulator and the sintered body had a uniform composition. The sintered body was 15 mm in diameter and 0.5 m in thickness.
3kv / m in silicon oil at 120 ° C
In polarization processing was performed, and the piezoelectric characteristics were examined. as a result,
The electromechanical coupling coefficient Kp shows a high value of 0.65,
It became clear that the sintered body had excellent piezoelectric properties.

For comparison, a heat treatment was performed using the same PZT compact sample and microwave heating apparatus as in Example 2 except that an insulator made of Al ₂ O ₃ was not used. As a result, at sintering at 1150 ° C. for 5 minutes, the sintering time was longer than in Example 1, so that the PZT sample as the sintered body and the SiC as the microwave absorber reacted, and the center of the sample surface was Had been transformed. In addition, elemental analysis in the thickness direction of the sample surface revealed that Si was diffused from the absorber. The piezoelectric characteristic of the sample of this comparative example is K
It was remarkably low at p = 0.3.

(Example 3) PLZT compact as a sintered body, SiC plate as a microwave absorber, and SiO _{2 as a} low loss body
Using a MgO plate for the tube and the insulator, microwave heating was performed using the same apparatus as in Example 2, and the performance of the obtained sintered body was evaluated. Hereinafter, a description will be given focusing on differences from the second embodiment.
PLZT powder is used as a sintered body, and the PLZT powder is subjected to a CIP treatment to obtain a PL having a diameter of 16 mm and a thickness of 2 mm.
A ZT green compact sample was prepared. The insulator has a diameter of 15m.
An MgO disk having a thickness of 2 mm and a thickness of 2 mm was used. Further, as a microwave absorber, SiC having a diameter of 15 mm and a thickness of 4 mm is used.
Using a plate, the whole was covered with a SiO ₂ tube. The sintering atmosphere was set to the atmosphere, and as a result of performing a heat treatment on the sintered body, the sintered body could be heated to 1150 ° C. or more without generating uneven heating or electric discharge. .
A sintered body having a true density of 8 g / cm ³ was obtained. Observation of the fracture surface and elemental analysis of the obtained sintered body revealed that the sintered body had no element diffusion from the insulator and had a homogeneous structure.

[Brief description of the drawings]

FIG. 1 is used in a microwave sintering method according to a first embodiment.
Is a schematic diagram of a microwave heating apparatus are.

FIG. 2 is a diagram showing a microwave sintering method according to a second embodiment.
1 is a schematic view of a portion of a microwave heating device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Microwave generation means 2 Cavity resonator 3 Waveguide 4 Coupler 6 Sintered body 7 Microwave absorber 8 Low loss body 11 Insulator

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-B-59-25937 (JP, B2) (58) Fields investigated (Int. Cl. ⁷ , DB name) C04B 35/64

Claims (3)

(57) [Claims] Transmission and microwave generating means for generating 1. A microwave, a cavity resonator for sinter heating and the object to be sintered body, the microwave from the microwave generation means to said cavity resonator A waveguide that introduces microwaves from the waveguide into the cavity to maintain resonance in the cavity, and a sintering body disposed in the cavity. using a microwave heating device having a body supporting means, easily absorb microwave large dielectric loss factor, and the <br/> temperature dependence of the dielectric loss factor than the sintered body is small microwave absorbing The body and the body to be sintered were in direct or indirect contact
In this state, the low-loss body is disposed on the sintered body supporting means, and a low-loss body having a small dielectric loss factor and easy to transmit microwaves is placed in front.
The sintered body and at least a part of the microwave absorber are
It is placed over a non-contact microphone, characterized by sintering the irradiated object sintered body microwaves to the object to be sintered body
Method of sintering by wave . 2. Sintering by microwave according to claim 1.
The method, wherein between the object to be sintered body the microwave absorber, the object to be sintered body and reacting with the microwave absorber <br/> flame rather, and dielectric loss factor is small insulator Place the said sintered
In a state where the body and the microwave absorber are in indirect contact with each other.
Sintering method by microwaves, characterized in that the Hare. 3. Sintering by microwave according to claim 1.
The method, wherein the object to be sintered and the microwave absorber are
Both are plate-shaped and are heated and sintered in a laminated state
A sintering method using microwaves.