JP2928605B2 - Ceramic sintering method - Google Patents

Description

Description: BACKGROUND OF THE INVENTION The present invention relates to a method for sintering ceramics, and more particularly, to the occurrence of uneven heating of ceramics having a large temperature dependence of a dielectric loss factor (ε _r tan δ). And a method of sintering uniformly.

[Prior art and its problems] Conventionally, as a method of sintering ceramics, there is a sintering method using microwave energy. As a heating furnace used for the microwave sintering, an oven system such as a microwave oven is used. In the oven, there are a large number of electromagnetic field modes, in which the object to be heated is arranged, so that a non-uniform electric field causes a temperature distribution in the object to be heated, causing uneven heating. In addition, since heating is performed in a large number of electromagnetic field modes, energy efficiency is poor, and there is a disadvantage that heating cannot be performed to high temperatures.

As a method for solving the problems of the prior art, a microwave heating furnace (Japanese Patent Publication No. 59-25937) capable of heating an object to be heated uniformly and at a high temperature with a small input has been proposed. As shown in FIG. 2, this microwave heating furnace is made of a material such as alumina porcelain having a high heat resistance and a large thermal conductivity, and has an inner container 95 for accommodating an object to be heated.
And an outer container 97 for housing the inner container 95 from a material having low microwave loss such as fire-insulating bricks or foamed alumina, and a high-filled space between the outer container 97 and the inner container 95. The object to be heated 94 is housed in a three-layered container comprising heat-producing particles 96 for heat generation and heat-generating particles 96 made of oxide or compound semiconductor such as zinc oxide or the like having a large microwave loss. Is configured to irradiate the particles 96 with microwaves. Accordingly, the inner container 95 having a high heat resistance and a large thermal conductivity for accommodating the object 94 to be heated.
In an outer container 97 made of a refractory heat insulating material having a small microwave loss in a form embedded with heat generating particles 96 made of a material having high heat resistance and a large microwave loss.
The configuration is such that the particles are heated by irradiating the particles with microwaves 97, so that the object to be heated 94 can be uniformly and efficiently heated to a high temperature.

However, in this conventional technique, the container is heated by microwave, and the object to be heated is heated by the radiant heat.
Dielectric loss factor (ε _r tan
This method cannot be said to be suitable for heating ceramics having a large δ). Further, since the object to be heated is not heated by microwaves, energy cannot be concentrated on the object to be heated, and heating can be performed only up to the heating temperature of the container. Furthermore, since an oven is used in the heating chamber, there is a problem that energy efficiency is low and the object to be heated cannot be heated to a high temperature.

On the other hand, as a method for concentrating microwave energy on an object to be heated, there is a method using a single-mode cavity resonator for generating a single (single) electromagnetic field mode.
According to this method, the object to be heated can be efficiently heated to the sintering temperature in a short time. However, this method
When ceramics having a large temperature dependence of the dielectric loss factor are heated, there is a problem that a temperature distribution due to a difference in the dielectric loss factor, a so-called runaway phenomenon, occurs in the object to be heated, and uneven heating occurs.

The inventors of the present invention have conducted intensive studies to solve the above-mentioned problems of the prior art, and as a result of repeated systematic experiments, the present invention has been accomplished.

[Object of the invention]

An object of the present invention is to provide a method for uniformly sintering ceramics having a large temperature dependence of a dielectric loss factor (ε _r tan δ) without causing uneven heating.

The present inventors have focused on the following with respect to the above-described problems of the related art. That is, first, attention was paid to the fact that when ceramics having a large temperature dependence of a dielectric loss factor and a low thermal conductivity were microwave-heated, uneven heating was caused in the ceramics due to a runaway phenomenon. As a result of investigating the cause, it was found that uneven heating was caused by a rapid change in the dielectric loss factor. Therefore, in order to prevent uneven heating caused by microwave heating of ceramics, which have a large temperature dependence of the dielectric loss factor, a substance having a smaller temperature dependence of the dielectric loss factor and a higher thermal conductivity than the ceramic object to be heated. The present invention has been accomplished by covering the object to be heated by the method described above and performing sintering without causing uneven heating.

Further, in order to heat the ceramic to be heated to a temperature higher than the sintering temperature, a single-mode cavity resonator is used, and the microwave absorber and the microwave absorber are placed at the maximum electric field strength in the resonator. By arranging and sintering the heated object, a homogeneous sintered material can be obtained without uneven heating.

[Description of the First Invention] The present invention provides a method of sintering ceramics according to the first invention, which comprises sintering a material made of a ceramic material by microwave irradiation. Absorbs well,
Irradiating microwaves in the cavity resonator for sintering through a microwave absorber made of a material having a smaller temperature dependence of the dielectric loss factor and a higher thermal conductivity than the material to be sintered. Features.

Effects and Effects of the Invention According to the method for sintering ceramics of the present invention, ceramics having a large temperature dependence of the dielectric loss factor (ε _r tan δ) can be uniformly sintered without causing uneven heating.

The mechanism by which the above-described effects can be obtained by this ceramic sintering method is not necessarily clear yet, but is considered as follows.

That is, the method for sintering ceramics of the first invention is as follows.
In a method of sintering a material to be sintered made of a ceramic material by microwave irradiation, the material to be sintered has a good microwave absorption, a temperature dependence of a dielectric loss factor smaller than that of the material to be sintered, and The cavity is sintered by irradiating microwaves in the cavity resonator via a microwave absorber made of a material having a high thermal conductivity.

The dielectric loss factor of ceramics is generally low at normal temperature and high at high temperature. That is, the higher the temperature, the better the absorption of microwaves. The temperature until sintering is in the low temperature range,
Considering the medium temperature range and high temperature range separately, in the present invention,
In the low temperature range, the dielectric loss factor of the sintering material made of ceramics is still small, so that the outer microwave absorber is mainly heated. In the medium temperature range, uneven heating is likely to occur due to the large temperature dependence of the dielectric loss factor of the material to be sintered, but a part of the microwave energy is consumed by the microwave absorber, so that the material to be sintered cannot be heated. Sudden changes in the dielectric loss factor are reduced, and the heat conduction of the absorber eliminates non-uniform portions in the sample temperature, so that the material to be sintered can be heated uniformly and uneven heating can be prevented. In the high temperature range, the dielectric loss factor of the material to be sintered is higher than that of the microwave absorber, and the temperature dependence is also smaller. Can do
It seems that a uniform and dense sintered body can be obtained.

[Description of Second Invention] Hereinafter, a second invention which is a more detailed description of the first invention will be described.

The method for sintering ceramics according to the second invention is a method for sintering a material to be sintered comprising a ceramic material by microwave irradiation, wherein the material to be sintered is irradiated with microwaves through a microwave absorber. It becomes.

Microwave Absorber The microwave absorber used in the present invention is made of a material that absorbs microwaves well and has a smaller temperature dependency of a dielectric loss factor and a higher thermal conductivity than the material to be sintered. That this absorber, dielectric loss factor of the absorber in the microwave band _{(0.3~30GH Z) (ε r tanδ} ) is normal temperature at least 0.05, and a thermal conductivity of 0.03cal / cm · sec · ℃ than the It is desirable that the thermal conductivity is higher than the thermal conductivity of the sintered material and the heat resistance is 1500 ° C. or higher. Further, the temperature dependence of the dielectric loss factor of the absorber,
It is desirable that the temperature be in the range of room temperature to 800 ° C., that is, one digit or less, that is, 1/10 to 10 times. If the dielectric loss factor is less than 0.05, absorption of microwaves becomes poor, and it is difficult to reduce a sudden change in the dielectric loss factor of the material to be sintered. When the thermal conductivity is less than 0.03 cal / cm · sec · ° C., a temperature distribution occurs in the absorber itself, so that it is difficult to prevent uneven heating of the material to be sintered. If the thermal conductivity of the microwave absorber is smaller than the thermal conductivity of the material to be sintered, it is difficult to make the temperature distribution of the material to be sintered uniform. Further, when the value of the dielectric loss factor changes by more than one digit in the range from ordinary temperature to 800 ° C., the absorber itself is not suitable because the runaway phenomenon may cause uneven heating.

The characteristic conditions of these microwave absorbers depend on the type of the material to be sintered. That is, the temperature dependency of the dielectric loss factor of the microwave absorber is smaller than that of the material to be sintered, and the thermal conductivity needs to be larger than that of the material to be sintered. If the temperature dependence of the dielectric loss factor of the microwave absorber is greater than that of the material to be sintered, the runaway phenomenon is more likely to occur in the microwave absorber, which promotes uneven heating of the material to be sintered. There is a fear. In addition, when the microwave absorber has a lower thermal conductivity than the material to be sintered, the temperature distribution generated in the microwave absorber is transmitted to the material to be sintered, which promotes the temperature distribution of the material to be sintered. There is a risk of doing so.

In particular, the characteristic of the microwave absorber is the dielectric loss factor (ε _r tan
δ) is 0.5 or more at room temperature, and the thermal conductivity is 0.3 cal / cm · sec · ° C
As described above, when the heat resistance is 1800 ° C or more and the temperature dependency of the dielectric loss factor is one digit or less in the range from room temperature to 1500 ° C, the effect of preventing uneven heating is excellent, and the characteristics of the sintered body are affected. Since the effect can be exhibited without being performed, it is most desirable as the microwave absorber of the present invention.

Materials of the microwave absorber, silicon carbide (SiC), zinc oxide (ZnO), carbon (C), boron carbide (B ₄ C), boron nitride (BN), beryllium oxide (BeO), aluminum nitride (AlN) And the like. Among them, in particular, in the case of silicon carbide (SiC) and carbon (C), since the dielectric loss factor is large, the temperature dependency is small, the thermal conductivity is large, and the heat resistance is excellent, so that the microwave of the present invention is used. It is a suitable material for the absorber. When using carbon,
It is preferable to use it under a reducing atmosphere.

The shape of the microwave absorber is such that at least a part of the absorber comes into contact with the surface of the material to be sintered, and at least a part of the absorber also acts on the microwave earlier than the material to be sintered. And the heat conduction from the contact portion of the microwave absorber to the sintering material and / or the radiant heat from the microwave absorber in a heated state effectively makes the temperature of the sintering body uniform. Preferably, the shape is such that
It is appropriately determined depending on the shape and structure of the material to be heated. In addition,
When a microwave absorber having a large dielectric loss factor and a large thermal conductivity is used, the microwave absorber is placed in an appropriate gap (0.5 to 2 mm) without contacting the absorber with the material to be sintered. The temperature of the material to be sintered can be made uniform by the radiation heat of the wave absorber.

The microwave absorber preferably has a thickness of 0.1 to 2 mm. This is because when the thickness is less than 0.1 mm, the effect of the microwave absorber becomes smaller, and when the thickness is less than 2 mm.
If the temperature exceeds the range, most of the microwaves are absorbed by the microwave absorber, and the temperature of the material to be sintered does not rise and the energy efficiency deteriorates, which is not preferable.

Regarding the Sintered Material The sintered material that is the object of the present invention is a material that is preferably sintered by the method of the present invention. Particularly preferable (advantageous) material is that the temperature dependence of the dielectric loss factor is normal temperature. To 800 ° C
And / or a ceramic material having a thermal conductivity of 0.1 cal / cm · sec · ° C. or less. Such materials tend to have uneven heating when sintered by ordinary microwave heating.

The dielectric loss factor of the material to be sintered at room temperature is not particularly limited, but a material having a dielectric loss factor of 0.05 or less is preferable. The use of such a material is preferable because the microwave absorber is mainly heated in a low temperature region before the runaway phenomenon occurs, and the temperature distribution of the material to be sintered becomes more uniform.

Regarding the shape of the material to be sintered, the effect of the present invention can be obtained by using an appropriate microwave absorber suitable for the shape of the material to be sintered, regardless of the shape. In the case of a rod-shaped or rod-shaped one, it is advantageous because it can be uniformly sintered to the inside.

Others The heating chamber of the microwave heating and sintering apparatus for ceramics used in the present invention uses a single-mode cavity resonator,
The shape of the cavity and the electromagnetic field mode are selected according to the type and shape of the material to be sintered. That is, since the sintering temperature of ceramics (> 1200 ° C.) is extremely high, it is difficult to heat the ceramics to the sintering temperature in an oven system in which microwave energy is dispersed. On the other hand, the ceramic heating device used in the present invention uses a single-mode cavity resonator. Therefore, since only one electromagnetic field mode is generated in the cavity, if the material to be sintered is disposed at the maximum location of the electric field, the microwave energy is concentrated at the location, and sintering is performed in a short time. Can be rapidly heated to temperature. In the oven method in which many electromagnetic field modes are generated, temperature distribution due to electric field non-uniformity occurs in the ceramic sintering material, but in a single mode resonator, the electric field in the sintering material is substantially uniform. In addition, it is possible to prevent uneven heating due to uneven electric field.

The cavity resonator has a cylindrical type or a rectangular parallelepiped type,
A rectangular parallelepiped resonator is suitable for a rod-shaped sample, and a cylindrical cavity resonator is suitable for a plate-shaped sample.

It is preferable that the material to be sintered is disposed at the maximum position of the electric field for effective sintering and parallel to the direction of the electric field for uniform sintering.

The sintering atmosphere is preferably in the air or oxygen when the material to be sintered is an oxide-based ceramic, and preferably in a reducing atmosphere when the material is a non-oxide ceramic.

In addition, by rotating the material to be sintered during microwave heating, the distribution of the electric field and the dielectric loss in the sample can be relaxed, which is more effective in preventing the occurrence of uneven heating.

By doing as above, in the related art, the temperature dependence of the dielectric loss factor is large, and when the sintering material made of ceramics having a low thermal conductivity is microwave-sintered, uneven heating occurs. By using a microwave absorber, sintering can be performed without causing uneven heating, and a uniform and dense sintered body can be obtained. Also,
By using a single-mode cavity resonator, the material to be sintered can be heated to the sintering temperature in a short time.

As a result, microwave sintering of ceramics having a large temperature dependence of the dielectric loss factor such as functional ceramics was made possible, and sintering in a short time and with high energy efficiency became possible.

〔Example〕

 Hereinafter, examples of the present invention will be described.

First Example PZT powder was used as a material to be sintered, and a silicon carbide (SiC) tubular body was used as a microwave absorber. Microwave irradiation was performed to produce a ceramic sintered body, and performance evaluation of the sintered body was performed. The test was performed. FIG. 1 shows a schematic cross-sectional view, partly omitted, schematically showing the ceramic sintering apparatus used in this process.

A ceramic sintering apparatus used for sintering ceramics according to the present embodiment will be briefly described with reference to FIG. The ceramic sintering apparatus 1 of this embodiment includes a microwave generating means 2 for generating microwaves, a cavity resonator 3 for heating and sintering a rectangular parallelepiped material, A waveguide 4 for transmitting microwaves from the generation means 2 to the cavity resonator 3, and a coupling window 5 for introducing microwaves from the waveguide 4 to the cavity resonator 3 and maintaining the resonance of the cavity resonator 3 And a support rod 6 made of quartz or sapphire that supports the material 9 to be sintered inserted into the cavity resonator 3 and a chuck 7 that fixes the support rod 6.

First, PZT powder was used as the sintering material 9 and subjected to CIP treatment to prepare a PZT sample of φ5 × 7 mm.

Next, the material to be sintered 9 is inserted into the support rod 6 of the cavity resonator 3, and the microwave absorber 8 made of a SiC tube having an inner diameter of 5 mm, an outer diameter of 8 mm and a length of 8 mm is further placed on the material to be sintered. 9 so as to cover it. The PZT sample 9 was arranged at the maximum electric field in the cavity 3 so as to be parallel to the direction of the electric field.

Next, a heat treatment was performed on the material to be sintered. Resonance frequency
2.45 GHz _Z, about 100W microwave power, an electromagnetic field mode in the resonator and TE _103, the sintering atmosphere was in the air.

As a result, it was possible to heat to 1100 ° C. or more without causing uneven heating, and a sintered body having 90% or more of the theoretical density was obtained at 1100 ° C. for 5 minutes. The obtained sintered body is
As a result of the fracture surface observation test, it was found that the sintered body had a homogeneous structure.

For comparison, a comparative test was performed using the same sintering material and ceramic sintering apparatus as in this example except that a microwave absorber made of a SiC tube was not used. As a result, when microwave heating was applied, uneven heating occurred at about 300 ° C., and the center of the PZT sample was locally heated, cracking occurred from the center, and sintering was not possible.

Next, in order to examine the temperature dependence of the dielectric loss factor of the PZT sample and the SiC tube, measurement tests of the dielectric loss factor were respectively performed. The result is shown in FIG. In the figure, "A" is PZT
“B” indicates the dielectric loss factor of the SiC tube, and “B” indicates the dielectric loss factor of the sample. From Fig. 3, the temperature dependence of the dielectric loss factor of the PZT sample is significantly greater than that of the SiC tube, and the PZT sample alone causes uneven heating due to the runaway phenomenon. , The absorption of a part of microwave energy was reduced and the temperature distribution of the PZT sample was reduced by heat conduction.Thus, it was possible to prevent uneven heating and obtain a homogeneous sintered body. It seems to have been.

Second Embodiment Barium titanate (BaTiO ₃ ) powder was used as a material to be sintered.
Using a silicon carbide (SiC) plate as a microwave absorber, a ceramic sintered body was manufactured by irradiating microwaves, and a performance evaluation test of the sintered body was performed. FIG. 4 is a schematic cross-sectional view, partly omitted, schematically showing the ceramic sintering apparatus used in this process. Hereinafter, the first
The following description focuses on the differences from the embodiment.

The ceramic sintering apparatus 21 used for sintering the ceramics of this embodiment will be briefly described with reference to FIG. Cavity resonator 23 used as a cylindrical, a resonance frequency 6GH _Z, about 150W microwave power, an electromagnetic field mode in the resonator and the TE _111. Reference numeral 22 denotes a microwave generation unit, and reference numeral 25 denotes a coupling unit for introducing microwaves to maintain resonance of the cavity. In the cavity 23, a material 29 to be sintered and / or a microwave absorber
28, and a support tube 26 on which the support tube 26 is fixed.
A turntable 27 that allows the 29 to rotate is provided.

First, as the sintered material 29, using a BaTiO ₃ powder was prepared BaTiO ₃ samples of 20 mm in diameter × 2 mm is subjected to CIP treatment.

The SiC plate used for the microwave absorber was φ22 mm × 1 mm, and the sintering atmosphere was air.

Next, the upper and lower sides of the BaTiO ₃ sample as a material to be sintered were sandwiched between SiC plates of a microwave absorber, and were placed at the position of the maximum electric field in the cavity resonator 23 by a support tube 26 made of quartz. The material to be sintered is rotated by a rotary table provided on the bottom surface in the resonator so that the electric field in the sample becomes uniform.

 Next, a heat treatment was performed on the material to be sintered.

As a result, it was possible to heat to 1300 ° C. or more without causing uneven heating, and a sintered body having 90% or more of the theoretical density was obtained at 1300 ° C. for 5 minutes. The obtained sintered body is
As a result of a fracture surface observation test, it was found that the sintered body had a homogeneous structure. In this embodiment, the electric field distribution has a circumferential direction.
Although the _T111 mode was used, it was possible to prevent the formation of a temperature distribution on the sample by rotating the sample on the table, and synergistically with the uniformization of the temperature distribution of the material to be sintered by the microwave absorber. It is considered that the non-uniform heating was prevented by acting to realize more uniform heating.

For comparison, a comparative test was performed using the same sintering material and ceramic sintering apparatus as in this example except that the microwave absorber was not used. As a result, when heated by microwaves, uneven heating occurred at about 200 ° C., and the side or bottom surface of the BaTiO ₃ sample was locally melted, and many cracks were generated and sintering could not be performed.

As is clear from the above, while applying microwaves through the microwave absorber, and rotating and sintering the material to be processed, effective absorption of part of microwave energy and heat conduction to BaTiO It is considered that since the temperature distribution of the _three samples could be made smaller, uneven heating could be prevented and a more homogeneous sintered body could be obtained.

Third Embodiment Using a silicon nitride (Si ₃ N ₄ ) powder as a material to be sintered and a carbon (C) tubular body as a microwave absorber, microwave irradiation is performed using the same apparatus as in the first embodiment. Then, a ceramic sintered body was manufactured, and a performance evaluation test of the sintered body was performed. Hereinafter, a description will be given focusing on differences from the first embodiment.

As a material to be sintered, Si ₃ N ₄ powder was subjected to CIP treatment to prepare a Si ₃ N ₄ sample of φ5 × 6 mm. In addition, a carbon tube having an inner diameter of 5 mm, an outer diameter of 7 mm, and a length of 7 mm was used as a microwave absorber. The arrangement structure and various conditions of the material to be sintered and the microwave absorber were the same as in the first embodiment.

As a result of heating the material to be sintered in nitrogen (N ₂ ) gas, it is possible to heat it to 1700 ° C or more without generating uneven heating, and to achieve a theoretical density of 90% at 1700 ° C for 10 minutes. %
The above sintered body was obtained. As a result of a fracture surface observation test, the obtained sintered body was found to be a sintered body having a homogeneous structure.

[Brief description of the drawings]

FIG. 1 is a partially omitted schematic sectional view schematically showing a ceramic sintering apparatus used in the first embodiment, FIG. 2 is a sectional view showing the concept of a conventional microwave heating furnace, and FIG. FIG. 4 is a diagram showing the temperature dependence of the dielectric loss factor of the material to be sintered and the microwave absorber in one embodiment, and FIG. 4 is a partial illustration schematically showing the ceramic sintering apparatus used in the second embodiment. It is an outline sectional view. 1, 21 ceramic sintering device 2, 22 microwave generating means 3, 23 cavity resonator 8, 28 microwave absorber 9, 29 material to be sintered

────────────────────────────────────────────────── ─── Continuing from the front page Examiner Yuki Yamada (56) References JP-B-59-25937 (JP, B2) (58) Fields investigated (Int. Cl. ⁶ , DB name) C04B 33/32 C04B 35 / 64

Claims (1)

(57) [Claims] 1. A method for sintering a ceramic material by irradiating a material to be sintered with microwaves, wherein the material to be sintered has a good microwave absorption and a temperature of a dielectric loss factor higher than that of the material to be sintered. A method for sintering ceramics, comprising irradiating microwaves in a cavity resonator and sintering through a microwave absorber made of a material having low dependence and high thermal conductivity.