JP2006347824A - Combustion synthesis method and oxide-based ceramic - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a combustion synthesis method for obtaining an oxide-based ceramic, capable of efficiently removing gases or sublimates as by-products from the inside of a main product when performing a combustion synthesis reaction and thus reducing the influence exerted from a reactor or the like, and also to provide the oxide-based ceramic obtained by the method. <P>SOLUTION: This combustion synthesis method generating gases or sublimates as by-products comprises a raw material preparation process for housing the mixture of a plurality of materials as elemental sources composing a main product in a reactor and an ignition process for igniting and reacting the mixture in the reactor. In the raw material preparation process, the mixture is housed in the reactor after being compressed as a material-compressed body. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a combustion synthesis method for producing oxide-based ceramics, particularly a reaction-based combustion synthesis method for generating by-products such as gas and sublimates during combustion synthesis, and oxide-based ceramics produced by the method. About.

Oxide ceramics (general formula ABO ₃ ) are used in various applications because they have excellent dielectric properties, superconductivity, proton conductivity, etc., depending on the combination of A and B in the general formula. Typical dielectric ceramics include BaTiO ₃ used for ceramic capacitors, PZT (PbZrO ₃ -PbTiO ₃ ), which is the main material of piezoelectric and pyroelectric ceramics, and high-temperature proton conductive oxidation for fuel cells. Examples thereof include SrCeO ₃ , BaCeO ₃ , CaZrO ₃ , SrZrO ₃ , and BaZrO ₃ .

Conventionally, in order to synthesize ceramics as described above, external heating must be performed using a furnace at about 1000 ° C. to about 2000 ° C. For this reason, the synthesis of ceramics requires enormous energy and a large heating mechanism, which increases the manufacturing cost.
As a manufacturing method without external heating, synthesis of ceramic powder by combustion synthesis has been proposed (see Patent Document 1). The combustion synthesis method is a method of synthesizing substances in a chain manner by utilizing a large amount of chemical heat reaction released at the time of compounding without requiring external heating.
In the manufacturing method according to Patent Document 1, three kinds of raw materials, ie, one kind of metal oxide and two kinds of different metal elements, are used as starting materials, and two kinds of intermetallic compounds or non-oxide ceramics and oxide ceramics are synthesized. is doing. For example, after mixing nickel oxide powder, aluminum powder, and alumina powder into a circular shaped product using a die press, the product is placed on a graphite plate, stored in a high-pressure reaction vessel, and the shaped product in an argon atmosphere. The oxidation reaction of aluminum powder is induced by igniting the upper end surface of the metal, and the combustion reaction proceeds in a chain while reacting with the excessively added aluminum of the reduced nickel to synthesize NiAl. As a result, an ingot of NiTi that is one of intermetallic compounds can be manufactured without external heating.
Moreover, in the said conventional manufacturing method, the graphite board is used as a part of material of a manufacturing apparatus. In many cases, a graphite crucible is used instead of the graphite plate (see, for example, Patent Document 2). As shown in FIG. 5, the mixed powder 1 serving as a reaction raw material is spread on the crucible 2 in a powder bed shape. Compressed after laying down, etc.

However, with the combustion synthesis method of Patent Document 1, oxide ceramics cannot be obtained. On the other hand, an oxide-based ceramic can be obtained by using an oxygen supply source and a heat source as reaction raw materials, but the mixed powder 1 of oxide-based raw materials is accommodated in a crucible 2 as shown in FIG. When the combustion synthesis reaction is performed, the mixed powder 1 and the graphite crucible 2 come into contact with most of the crucible inner surfaces 2a and 2b, so that the contact area is large, and unnecessary carbides may be generated at the contact portion.
Further, in the reaction system that generates gas or sublimate as a by-product, when the mixed powder 1 as the reaction raw material is accommodated in the crucible 2 as shown in FIG. Since it propagates concentrically and proceeds in a chain, gas or sublimate is difficult to escape from the bottom inner surface 2b or side inner surface 2a of the crucible 2 which is the end, and a part of the gas or sublimate may be taken into the composite.
Japanese Patent Laid-Open No. 5-9009 JP 2000-16804 A

  The present invention has been made in order to cope with such problems, and in a combustion synthesis method for obtaining an oxide-based ceramic that generates a gas or a sublimate as a by-product, the combustion synthesis reaction is performed at the time of the combustion synthesis reaction. An object of the present invention is to provide a combustion synthesis method capable of efficiently removing the by-product from the main product and reducing the influence from a reaction apparatus and the like, and an oxide ceramic obtained by the method.

  The combustion synthesis method of the present invention includes a raw material preparation step in which a mixture obtained by mixing a plurality of raw materials serving as an element source constituting a main product is accommodated in a reactor, and an ignition in which the mixture in the reactor is ignited and reacted. Comprising a step of generating a gas or a sublimate as a by-product, wherein the raw material preparation step compresses the mixture into a raw material compressed body and then stores the compressed material in the reactor. It is a process to perform.

In the raw material preparation step, the raw material compressed body is placed in the reactor via a spacer material of the same type as the main product. The same type includes not only the same composition but also a similar composition.
The raw material compressed body is characterized in that a communication hole is formed inside the compressed body.

  The ignition step is a step of igniting a predetermined amount of the mixture in contact with the raw material compressed body.

The oxide-based ceramic of the present invention is an oxide-based ceramic that uses a mixture of an oxide-based material and a material that serves as an oxygen supply source as a reaction material, and is obtained by the combustion synthesis method described above. And
The oxide-based raw material is a raw material including an element source that constitutes an oxide-based ceramic that is a main product.

The combustion synthesis method of the present invention reduces the contact area between the reaction raw material and the reaction device such as a graphite crucible after the mixture as the reaction raw material is compressed into a raw material compressed body and then stored in the reaction device. And the generation of carbides in the portion can be reduced. Since it mounts via a spacer material, the said contact area can further be reduced. In addition, by making the spacer material the same material as the main product, the composition of the main product after combustion synthesis is not affected.
By using a compression body as described above, and further using a spacer material, the contact area between the reaction raw material and the reactor is reduced, so even if a gas or sublimate is generated as a by-product during combustion synthesis, These by-products can be efficiently removed from the surface of the raw material compact. Moreover, since a communicating hole is previously formed in the raw material compression body, a by-product can be eliminated more efficiently.

  Since the oxide-based ceramic of the present invention is obtained by the above combustion synthesis method, it is obtained as a sintered body having a high purity with few impurities such as by-products and carbides.

The combustion synthesis method of the present invention is a combustion chemical reaction system in which gas or sublimate is generated as a by-product, and is a method for obtaining oxide ceramics as a main product.
Examples of the reaction system in the combustion synthesis method of the present invention, wherein (a) only gas is generated as a by-product, for example, metal powder containing a group 4 element, carbonate of an element containing a group 2 element, and As a reaction system using a reaction raw material containing at least a peroxide of an element containing a group 2 element, (b) generating a gas and a sublimate as a by-product, for example, a metal powder containing a group 4 element, A reaction system using a reaction raw material containing at least a carbonate of an element containing a group 2 element and sodium perchlorate can be mentioned.

The reaction system (b) may be a group 4 metal powder, a group 2 carbonate, and a group 2 peroxide alone, or a reaction product obtained by adding a group 4 metal oxide to the piezoelectric. It is preferable because of its excellent properties, dielectric properties, cost and the like. Moreover, each reaction raw material is mix | blended in the predetermined ratio which satisfy | fills the chemical reaction formula in a combustion synthesis method. For example, when synthesizing an oxide-based ceramic such as barium titanate (BaTiO ₃ ) or calcium titanate (CaTiO ₃ ) as the main product, it is generated according to the following chemical reaction formula. The Group 4 metal powder, the Group 2 carbonate, and the Group 2 peroxide are blended in amounts corresponding to respective molar masses necessary for the following reaction. In the reaction system, carbon dioxide (CO ₂ ) is generated as a by-product.
In addition, when blending Group 4 metal oxides, blend them at a rate that allows the adiabatic flame temperature to be maintained at 1500 ° C or higher. The adiabatic flame temperature can be lowered by increasing the proportion of the Group 4 metal oxide.

Ti + 2TiO ₂ + BaCO ₃ + 2BaO ₂ → 3BaTiO ₃ + CO ₂ ↑
Ti + 2TiO ₂ + CaCO ₃ + 2CaO ₂ → 3CaTiO ₃ + CO ₂ ↑
Ti + 2TiO ₂ + BaCO ₃ + 2CaO ₂ → 3 (Ba _1/3 , Ca _2/3 ) TiO ₃ + CO ₂ ↑
Ti + 2TiO ₂ + CaCO ₃ + 2BaO ₂ → 3 (Ba _2/3 , Ca _1/3 ) TiO ₃ + CO ₂ ↑

The reaction system of (b) above is a group 4 metal powder, a group 2 carbonate, and NaClO ₄ alone, or the addition of a group 4 metal oxide to this, and the reaction product has excellent detergency. It is preferable because of its excellent piezoelectricity and dielectric properties.
The reaction raw materials are blended at a predetermined ratio. In the combustion synthesis reaction, for example, in the case of strontium titanate (SrTiO ₃ ), the reaction raw materials are produced according to the following chemical reaction formula. Each reaction raw material is blended in an amount corresponding to the molar mass required for the reaction between the Group 4 metal powder and the Group 2 carbonate, but the oxygen generating substance can be blended in an amount greater than the molar mass necessary for the reaction.

Ti + SrCO ₃ + 0.5NaClO ₄ → SrTiO ₃ + CO ₂ ↑ + 0.5NaCl

The raw material preparation step in the combustion synthesis method of the present invention includes a step of mixing a plurality of genus powders, carbonates, peroxides, etc., which are element sources constituting the main product such as the oxide ceramics, And the step of accommodating the obtained mixed powder in a reactor.
Examples of the reaction apparatus include a crucible, a reaction vessel similar to a crucible, a setter, and the like.

In the combustion synthesis method of the present invention, the step of mixing a plurality of raw materials serving as the element source constituting the main product was mixed at an atomic ratio capable of forming the main product in the reaction system of (a) or (b) above. In this step, the raw materials are mixed using a known mixing device such as a ball mill or a mortar.
In the reaction system (a) or (b), the shape of the metal containing a group 4 element as a reaction raw material is preferably a fine powder, and the specific surface area is 0.01 to ² m ² / g. A preferable specific surface area is 0.1 to 0.6 m ² / g because the combustion wave propagates and is easy to handle. When the specific surface area is less than 0.01 m ² / g, since the contact area between the metal powder that is a heat source and the peroxide that is an oxygen supply source is small, the combustion wave may not propagate and ceramics may not be synthesized. In addition, metal powders having a specific surface area exceeding 2 m ² / g are not preferable because they are extremely active and difficult to handle.
In the present invention, the specific surface area of the metal powder is a value measured by the BET method.

Moreover, even if the average particle diameter of the metal fine powder is the same, a difference in reactivity is recognized when the specific surface area is different. That is, when a metal powder having a specific surface area larger than that of a spherical shape was used, the combustion synthesis reaction proceeded more rapidly. As the shape having a large specific surface area, there are a particle having a plurality of irregularities formed on the surface of a spherical particle, a particle having an irregular shape as a whole, or a combination thereof.
The average particle size that can be used in the present invention is 150 μm or less, preferably 0.1 to 100 μm. If it exceeds 150 μm, mixing with other raw materials becomes insufficient, and combustion waves may not propagate. An image analysis method is preferable as a method for measuring the average particle diameter of particles having irregularities formed on the surface or an irregular shape.

The process of accommodating the mixed powder as the reaction raw material in the reaction apparatus (graphite crucible) will be described below.
A mixed powder storage method according to an embodiment of the present invention will be described with reference to FIG. FIG. 1 is a reference cross-sectional view of a reaction raw material and a crucible.
As shown in FIG. 1, the mixed powder 1 is compressed into a raw material compressed body 1 a and then placed in a graphite crucible 2. The raw material compressed body 1a has a so-called pellet shape, and a polymer material such as polyvinyl alcohol can be used as a binder during compression. After processing into a pellet shape, by placing in the crucible 2, the contact area with the crucible can be greatly reduced as compared with the case where the mixed powder is filled as shown in FIG.
The shape of the raw material compressed body 1a is not particularly limited to the shape shown in FIG. 1, and can be any shape. Since generation of unnecessary carbides can be suppressed, and elimination of by-products is facilitated, the shape is preferably a shape that minimizes the contact area with the side inner surface 2a and the bottom inner surface 2b of the crucible 2. . Specifically, when the crucible 2 has a cylindrical shape, contact with the side inner surface 2a can be avoided by making the disc shape smaller in diameter than the inner diameter, and by curving the bottom portion, etc. The contact area with the inner surface 2b can be reduced.

Moreover, as shown in FIG. 2, the raw material compression body 1a can be mounted in the crucible 2 through the spacer material 4 of the same kind as the main product. By interposing the spacer material 4 between the bottom inner surface 2b of the crucible 2 and the raw material compressed body 1a, contact between the crucible 2 and the raw material compressed body 1a can be completely prevented. Further, the contact area between the bottom inner surface 2b and the crucible 2 can be reduced as compared with the case where the raw material compressed body 1a is simply placed in the crucible 2.
The spacer material 4 has the same or similar composition as the main product to be obtained by combustion synthesis. By using the same material as the main product, the composition of the main product after combustion synthesis is not affected.

As described above, the contact area between the raw material compressed body 1a, which is a reaction raw material, and the crucible 2 is reduced by using a pellet and further using a spacer material. As a result, the gas (CO ₂ ) and the sublimate (NaCl) generated in the reaction system (b) or (b) can be efficiently removed from the surface of the raw material compression body sequentially as the combustion synthesis reaction proceeds. Further, the contact area with the crucible 2 is small, and there is little risk of mixing carbides in the main product.

Further, by forming a communication hole in the interior in advance during the processing of the raw material compressed body 1a, it is possible to more efficiently exclude gas and sublimate as by-products. A pellet having communication holes can be prepared by preforming at a pressure of 0.5 to 1.5 ton / cm ² . Further, by forming a plurality of pins on the lower punch (convex type), and having a plurality of pin holes on the upper punch (concave type), and compressing and molding, it is possible to provide through holes on the upper and lower sides.

The raw material compact 1a is ignited and reacted by a combustion synthesis method. Regarding the conditions of the combustion synthesis method, the adiabatic flame temperature of the reaction system is 1500 ° C. or higher. This is because the combustion wave propagates at 1500 ° C. or higher.
Combustion synthesis is performed in a chamber, and the atmosphere is preferably a rare gas atmosphere such as helium (He), neon (Ne), argon (Ar), or krypton (Kr). Note that a nitrogen gas, carbon dioxide atmosphere, or the like can be used as long as the dielectric properties of the reaction product are not deteriorated. Also, oxygen gas can be used if the oxygen partial pressure can be controlled.

The ignition process in which the mixture is ignited and reacted is not particularly limited as long as it is a method that enables the raw material compressed body 1a to generate heat. A method of energizing and igniting a carbon film to generate heat and making it a heat source and bringing it into contact with the mixed powder and igniting and generating heat is preferable because of excellent handling properties. The ignition point 3 may be any of the upper surface of the raw material compressed body 1a shown in FIGS. 1 and 2 and the lower surface shown in FIG.
As shown in FIG. 5, since the combustion wave in the combustion synthesis method propagates concentrically from the ignition point, it is better that there are few factors inhibiting the discharge of by-products on the surface side corresponding to the propagation end. Therefore, as the ignition point 3, it is preferable to ignite from the lower surface of the raw material compressed body 1a as shown in FIG. 3 so that by-products can be discharged from the upper surface or the like.

  When the carbon film is ignited, as shown in FIG. 4, the mixed powder 1 which is a reaction raw material before compression is placed on the raw material compressed body 1a, and the carbon film 5 is brought into contact with the mixed powder 1 to be ignited. preferable. This is because, in the combustion synthesis method of the present invention, the mixed powder 1 that is a reaction raw material is processed into the raw material compressed body 1a, and therefore, it is difficult to ignite when directly contacted by a carbon film.

Through the above raw material preparation step and ignition step, the chemical reaction proceeds simultaneously and frequently from the ignition portion without requiring external heating, and a reaction product is obtained. The combustion synthesis reaction is completed in about 1 to 60 seconds.
Oxide ceramics can be obtained by pulverizing the sintered body as the reaction product and sintering the powder. When sintering, a molding binder such as polyvinyl butyral can be blended. Sintering conditions include a condition of forming at a pressure of 10 to 100 MPa and firing at a temperature of 1200 to 1500 ° C. in an air atmosphere.
Moreover, in order to further stabilize the crystal structure of the synthetic powder obtained by the combustion synthesis and to remove a trace amount of impurities, it is possible to calcine at 900 to 1100 ° C.
In the reaction system (a), only carbon dioxide gas is generated as a by-product, but in the reaction system (b), NaCl is generated. In this case, the obtained sintered body is pulverized and then washed to remove NaCl.

  Oxide ceramics obtained by the above combustion synthesis method have few dielectric impurities, are close to the theoretical density, and have excellent dielectric properties, so dielectric antennas, capacitors, dielectric resonators, filters, pressure It can be used for sensors, ultrasonic motors, etc.

Example 1
As the compounding raw material, 1 mol of Ti metal (specific surface area 0.3 m ² / g), 1 mol of SrCO ₃ and 0.5 mol of NaClO ₄ were used. These raw materials were mixed for 5 hours using a ball mill. The mixed powder (100 g) was pressed and solidified into a pellet, and placed in a graphite crucible in the chamber of the synthesis apparatus. A carbon film for ignition was brought into contact with the upper surface of the pellet to ignite. Combustion waves propagated by ignition, and synthetic powder was obtained by combustion synthesis. The chamber was filled with argon (Ar) gas and the internal pressure was 0.1 MPa.

Example 2
A mixed powder was obtained using the same blending raw material and synthesis apparatus as in Example 1. The mixed powder (100 g) was pressed and formed into pellets, and the pellets were placed in a graphite crucible in the chamber of the synthesizer, with bulk SrTiO ₃ interposed as a spacer material. A carbon film for ignition was brought into contact with the upper surface of the pellet to ignite. Combustion waves propagated by ignition, and synthetic powder was obtained by combustion synthesis.

Example 3
Combustion synthesis was performed under the same conditions as in Example 2 except that an ignition carbon film was brought into contact with the lower surface of the pellet and ignited to obtain a synthetic powder.

Example 4
A mixed powder was obtained using the same blending raw material and synthesis apparatus as in Example 1. The mixed powder (100 g) was preformed at a pressure of 1.0 ton / cm ² to produce pellets having communication holes. The pellets were placed in a graphite crucible in a chamber of a synthesis apparatus with bulk SrTiO ₃ interposed as a spacer material. A carbon film for ignition was brought into contact with the upper surface of the pellet to ignite. Combustion waves propagated by ignition, and synthetic powder was obtained by combustion synthesis.

Comparative Example 1
As the compounding raw material, 1 mol of Ti metal (specific surface area 0.3 m ² / g), 1 mol of SrCO ₃ and 0.5 mol of NaClO ₄ were used. These raw materials were mixed for 5 hours using a ball mill. The mixed powder (100 g) was spread in a graphite crucible in the chamber of the synthesizer, and a carbon film for ignition was brought into contact with a part of the mixed powder to ignite. Combustion waves propagated by ignition, and synthetic powder was obtained by combustion synthesis.

  The synthetic powders obtained in Examples 1 to 4 and Comparative Example 1 were pulverized using an alumina mortar to obtain ceramic powder having an average particle diameter of 1 μm. The crystal phase of each ceramic powder was identified using an X-ray diffractometer (XRD). The results are shown in Table 1.

As shown in Table 1, high purity ceramics (SrTiO ₃ ) with few unreacted parts and the like were obtained in each example. Further, in Comparative Example 1, the unreacted part including TiC was 3% by weight. TiC or the like was also detected in Example 1, but if it is 0.3% by weight or less, it is considered that the dielectric properties and the like of the ceramic are not affected.

  Oxide ceramics obtained by the combustion synthesis method of the present invention have excellent dielectric properties with less impurities mixed and close to the theoretical density, so that antennas, capacitors, resonators, pressure sensors, ultrasonic motors, etc. Can be suitably used in the field of electronic components.

It is a reference sectional view of a pellet-like reaction raw material and a crucible. It is a reference sectional view at the time of interposing a spacer member. It is a reference sectional view which illustrates the position of an ignition point. It is a reference sectional view of an ignition method. It is a figure which shows the mode of propagation of the combustion wave in a combustion synthesis method.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Mixed powder 1a Raw material compression body 2 Crucible 2a Side inner surface 2b Bottom inner surface 3 Ignition point 4 Spacer material 5 Carbon film

Claims (5)

Comprising: a raw material preparation step for containing a mixture of a plurality of raw materials serving as an element source constituting the main product in a reaction device; and an ignition step for igniting and reacting the mixture in the reaction device. A combustion synthesis method for generating gas or sublimate as a product,
The raw material preparation step is a step of compressing the mixture into a raw material compressed body and then storing the mixture in the reaction apparatus.   2. The combustion synthesis method according to claim 1, wherein, in the raw material preparation step, the raw material compressed body is placed in the reactor via a spacer material of the same type as the main product.   The combustion synthesis method according to claim 1 or 2, wherein the raw material compressed body is formed with a communication hole inside the compressed body.   The combustion synthesis method according to claim 1, wherein the ignition step is a step of igniting a predetermined amount of the mixture in contact with the raw material compressed body. It is an oxide-based ceramic using a mixture of an oxide-based material and a material serving as an oxygen supply source as a reaction material,
The oxide ceramics obtained by the combustion synthesis method according to any one of claims 1 to 4, wherein the oxide ceramics.

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