JP5327677B2 - BaTi2O5 ferroelectric ceramic manufacturing method - Google Patents

Description

The present invention relates to a BaTi ₂ O ₅ -based ferroelectric ceramic manufacturing method, a ferroelectric ceramic, a capacitor, and an FRAM, and in particular, a BaTi ₂ O ₅ -based ferroelectric ceramic that is excellent in formability using a general-purpose firing technique and It relates to the technology that provides the applied products.

The BaTi ₂ O ₅ single crystal exhibits a very high dielectric constant as disclosed in Japanese Patent No. 4051437 by the present inventor. Specifically, the ferroelectric phase transition temperature T _C is as high as 470 ° C., the dielectric constant in the b-axis direction at T _C is nearly 30000, and the dielectric loss is as low as 0.1 or less even at 500 ° C. In addition, since it does not contain lead in the composition, it has attracted attention as a substitute for PZT as a so-called lead-free ferroelectric material.

Here, the BaTi ₂ O ₅ single crystal obtained in the laboratory system has a size of only a few mm, and in order to improve practicality, it is desired to use ceramics. Barium titanate is known as a substance having various compositions, but generally, it needs to be fired at a temperature of 1300 ° C. or higher in order to be converted into ceramic.

However, since BaTi ₂ O ₅ is decomposed into BaTiO ₃ and Ba ₆ Ti ₁₇ O ₄₀ at 1150 ° C. or higher, conventionally, special techniques such as high-temperature and high-pressure reactions and spark / plasma firing have been used to make ceramics. There was a problem that it was necessary to use, and it was not possible to synthesize the ceramics simply and industrially. In addition, such special firing has a problem that only a simple shape such as a flat plate shape can be obtained, and the formability is poor.

Japanese Patent No. 4051437 JP 2001-163664 A Sea-Fue WANG et al, 'Properties of Hexagonal Ba (Ti1-xMnx) O3Ceramics: Effects of Sintering Temperature and Mn Content' Japanese Journal of Applied Physics Vol.46, No.5A, 2007, pp.2978-2983

That is, the problem to be solved is that, for example, a BaTi ₂ O ₅ type ferroelectric ceramic with good formability is obtained by using a general firing method such as applying temperature under normal pressure. Also, a point to provide a product which utilizes the BaTi ₂ O ₅ based ferroelectric ceramics.

The BaTi ₂ O ₅ type ferroelectric ceramic manufacturing method according to claim 1, wherein BaTi ₂ O ₅ powder is added with Mn as an auxiliary agent and fired at 1200 ° C. or higher to produce a BaTi ₂ O ₅ type ferroelectric ceramic. The most important feature is to obtain.

Incidentally, the BaTi ₂ O ₅ system means a composition in which decomposition does not occur (BaTi ₂ O ₅ is usually BaTiO ₃ and Ba ₆ Ti ₁₇ O ₄₀ at 1150 ° C. or higher. Decomposes and begins to dissolve at 1322 ° C). In other words, it means a single phase crystal type. In this sense, the BaTi ₂ O ₅ system includes a composition in which Mn is partially substituted with Ti or Ba. The powder means nanocrystal or fine powder. Such a powder can be obtained, for example, by a sol-gel method using an alkoxide.

Mn is not particularly limited as long as it does not alter barium titanate (does not affect firing) at a temperature of about 1200 ° C. as well as the metal itself, and may be a compound. For example, MnO ₂ can be mentioned. The melting point of MnO ₂ is 535 ° C. Examples of the amount of Mn added include an example in which 0.4 wt% of the raw material is added.

  The firing temperature is preferably 1250 ° C. or higher as will be described later.

The method according to claim 2 is the BaTi ₂ O ₅ based ferroelectric ceramic manufacturing method according to claim 1, wherein the real part of dielectric constant ε ′ ≧ when fired at 1250 ° C. or higher and the measurement frequency is 1 MHz. The main feature is to obtain a BaTi ₂ O ₅ type ferroelectric ceramic of 350.

  Note that ε ′ is a non-oriented value because the powder is fired as it is.

The method according to claim 3 is the BaTi ₂ O ₅ ferroelectric ceramic manufacturing method according to claim 1 or 2, wherein the sintered product is repeatedly pulverized, reshaped and fired, and the degree of sintering is 80. The main feature is that it is over%.

The ferroelectric ceramic according to claim 4 is a ceramic obtained by adding Mn as an auxiliary agent to BaTi ₂ O ₅ powder and firing it, and when the measurement frequency is 1 MHz, the dielectric constant real part ε '≧ 350 is the main feature.

  The capacitor according to a fifth aspect is characterized by using the ferroelectric ceramic obtained by the method according to the first, second or third aspect, or the ferroelectric ceramic according to the fourth aspect.

According to the present invention, it is possible to obtain a BaTi ₂ O ₅ ferroelectric ceramic with good formability using a general firing method. In addition, a product using a BaTi ₂ O ₅ ferroelectric ceramic can be provided.

It is the figure which showed the relationship between the mode of the surface of the baked product after 2 times of baking, and baking temperature. It is the figure which showed the relationship between the magnitude | size of the spot on the surface of a baked product, baking temperature, and the frequency | count of baking. It is the figure which showed the relationship between the sintering degree of baking products, baking temperature, and the frequency | count of baking. It is the figure which measured the XRD pattern of the baked product after baking once. It is the figure which investigated the relationship between the dielectric constant of the baked product baked twice at 1100 degreeC, temperature, and a measurement frequency. It is the figure which investigated the relationship between the dielectric constant of the baked product baked twice at 1150 degreeC, temperature, and a measurement frequency. It is the figure which investigated the relationship between the dielectric constant of the baked product baked twice at 1200 degreeC, temperature, and a measurement frequency. It is the figure which investigated the relationship between the dielectric constant of the baked product baked twice at 1250 degreeC, temperature, and a measurement frequency. It is the figure which accumulated and showed the relationship between the dielectric constant and temperature of the baked product in each baking temperature in case a measurement frequency is 1 MHz. It is the figure which showed the dielectric constant of the baked product which changed the addition amount of Mn and baked twice at 1250 degreeC.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
In the present invention, barium titanate having various composition forms generally has a ceramization temperature of 1300 ° C. or higher, and as a result of intensive studies by the inventors of the present application, when Mn is added, the firing temperature is as low as 50 ° C. to 100 ° C. However, it was made on the basis that it was found that it can be ceramicized and maintains its initial composition and also has ferroelectricity even if it is a composition that normally decomposes at this temperature and changes to another composition. Is. Below, it demonstrates in order of preparation of a nanoparticle by a sol-gel method, baking, and physical-property evaluation.

<Preparation of BaTi ₂ O ₅ nanoparticles by sol-gel method>
First, a raw material was prepared by a sol-gel method.
Ba (OC ₂ H ₅ ) ₂ (manufactured by High-Purity Chemical Laboratory: purity 99.0% or higher): Ti [OCH (CH ₃ ) ₂ ] 4 (manufactured by Kishida Chemical: purity 99.0% or higher) = 1: 2 As (molar ratio), dissolved in a mixed solvent of methanol (manufactured by Kanto Chemical Co., Inc .: purity 99%) and 2-methoxyethanol (manufactured by Kishida Chemical Co., Ltd .: purity 99% or more) in nitrogen gas, and having a concentration of 1.0 mol / L A precursor solution was made.

This was cooled to 0 ° C. and hydrolyzed by spraying distilled water while stirring.
Next, the precursor gel was produced by aging at 50 ° C. for 24 hours.
This was dried and crushed into a powder in a mortar.

The obtained powder was put into an alumina crucible and calcined at 650 ° C. for 12 hours in an electric furnace to obtain BaTi ₂ O ₅ . At this time, the temperature rising rate was 200 ° C./h. As will be described later, the obtained crystal was confirmed to be BaTi ₂ O ₅ by XRD pattern analysis.

<Addition and molding of auxiliaries>
Next, 0.4 wt% of MnO ₂ was added to the powder and the mixture was finely granulated by a wet ball mill. Specifically, 3.00 g of BaTi ₂ O ₅ raw material powder and 0.012 g of previously pulverized MnO ₂ (manufactured by SIGMA-ALDRICH: purity 99.999%) are put into a ball mill, and ethanol is placed in a container. After adding up to 7 minutes, 5 mmφ10 pieces and 2 mmφ50 pieces of zirconia balls were added, and the container was continuously rotated at a speed of 150 rpm for 24 hours. Thereafter, the content was dried at 100 ° C. and pulverized in a mortar to obtain a uniformly mixed powder (a mixed powder of BaTi ₂ O ₅ and MnO ₂ ).

  Next, 1 wt% of polyvinyl alcohol as a binder was added to the powder, put into a 5 mmφ mold, and pressed at 500 MPa for 10 minutes to produce pellets. The height of the pellet was about 4.5 mm and the weight was about 0.3 g.

<Baking>
The pellets were fired at 900 ° C., 1000 ° C., 1100 ° C., 1150 ° C., 1200 ° C., and 1250 ° C. for 5 hours. In the temperature raising process, a step of 600 ° C. × 4 hours was performed to remove the binder, and the temperature was raised to the set temperature.

  In the baked product, the clearer spots were confirmed as the calcination temperature was higher, so the pellets were pulverized, added again with 1 wt% of polyvinyl alcohol, molded using the same mold, and kept at the same temperature. And fired again. At this time, the temperature raising program was the same as that for the first firing, including the binder removal step.

<Physical property evaluation>
FIG. 1 shows the relationship between the state of the surface of the fired product after firing twice and the firing temperature. As shown in the figure, the spots become more conspicuous as the firing temperature is higher, but in the second firing, noticeable spots as in the first firing were not observed. FIG. 2 shows the relationship between the size of the spots, the firing temperature, and the number of firings. In addition, when it confirmed with the metal microscope, the magnitude | size of the spot which appeared at the time of the 1st baking was a magnitude | size of 80 micrometers-100 micrometers.

FIG. 3 shows the relationship between the degree of sintering, the firing temperature, and the number of firings. As shown in the figure, although the firing time was short or the voids due to binder dissipation were large, the degree of sintering was not large, but at 1150 ° C or higher, the degree of sintering exceeded 80% by firing twice. It can be confirmed that ceramics are being developed. In particular, it can be confirmed that when the sintering degree exceeds 85% at 1,200 ° C. or more and the degree of sintering exceeds 85%, it is almost 100% ceramicized. The degree of sintering was calculated from the pellet volume and weight based on the density of the BaTi ₂ O ₅ single crystal (5.2 g / cm ³ ).

Next, the XRD pattern after firing twice was measured. The results are shown in FIG. As shown in the figure, the intensity of the peak is slightly different, but it has a pattern almost the same as that of the raw material powder, that is, BaTi ₂ O ₅ crystal up to 1250 ° C., and does not decompose even at a temperature that should normally be decomposed. It was confirmed that the crystal form of was maintained.

  From the above, when Mn is used as an auxiliary agent, even a person skilled in the art, which is generally called the ceramicization temperature of barium titanate-based crystals, which is 50 ° C lower than 1300 ° C, can perform ceramicization at an unexpected temperature. In addition, it was confirmed that two surprising results were obtained that the ceramic could be produced without decomposition even at 1150 ° C. or higher, which is the decomposition temperature.

  Next, the relationship between the dielectric constant after firing twice, the temperature, and the measurement frequency was examined. FIG. 5 shows a 1100 ° C. fired product, FIG. 6 shows a 1150 ° C. fired product, FIG. 7 shows a 1200 ° C. fired product, and FIG. 8 shows a 1250 ° C. fired product. FIG. 9 also shows the relationship between the dielectric constant and the temperature of the fired product at each firing temperature when the measurement frequency is 1 MHz.

  As is apparent from the figure, when the firing temperature is 1200 ° C., Tc is 320 ° C. to 330 ° C., the real part ε ′ of the dielectric constant is 150 or more, and in particular, the firing temperature is 1250 ° C. In some cases, the real part ε ′ of the dielectric constant was 350 or more, and it was confirmed that good ferroelectricity was exhibited. This value obtained by a normal firing method at normal pressure (atmospheric pressure) is large.

  From the above, it was confirmed that a ceramic having ferroelectricity can be obtained by forming the raw material powder into an arbitrary shape and firing at 1200 ° C. or higher, preferably 1250 ° C. or higher. In the case of a single crystal, there is orientation, and ε ′ is 30000 in the b-axis direction, but ε ′ is about 350 because this method is formed by powder. However, it is possible to set ε ′ to about 1000 by appropriately giving a slight orientation by a conventionally known method.

Finally, the relationship between the amount of Mn added and the dielectric constant was examined. FIG. 10 is a graph showing the relationship between dielectric constant, temperature, and Mn addition amount. As a result of examining the fired product fired twice at 1250 ° C., it was found that Mn exhibits ferroelectricity when 0.2 wt% to 0.8 wt% is added. Although not shown, both, XRD pattern was a single phase crystal of BaTi ₂ O ₅ system.

As described above, according to the present invention, it is possible to obtain a BaTi ₂ O ₅ ferroelectric ceramic with good formability.

According to the present invention, it is possible to produce a capacitor in a series of steps, which is performed in a conventional production line, from raw slurry to a multilayer ceramic capacitor through a green sheet. Moreover, substitution of the PTC ceramics used for the thermistor is also possible. Moreover, application as FRAM is also possible.

Claims (5)

A method for producing a BaTi ₂ O ₅ ferroelectric ceramic, characterized in that BaTi ₂ O ₅ powder is added with Mn as an auxiliary agent and fired at 1200 ° C. or higher to obtain a BaTi ₂ O ₅ ferroelectric ceramic. And fired at 1250 ° C. or higher, BaTi ₂ according to claim 1, the measurement frequency, characterized in that to obtain a BaTi ₂ O ₅ based ferroelectric ceramics is the permittivity real part epsilon '≧ 350 when it is 1MHz O ₅ based ferroelectric ceramic manufacturing method. The method for producing a BaTi ₂ O ₅ ferroelectric ceramic according to claim 1 or 2, wherein the sintered product is pulverized, reshaped and fired repeatedly so that the degree of sintering becomes 80% or more. . Ferroelectric ceramics obtained by adding Mn as an auxiliary to BaTi ₂ O ₅ powder and firing, and having a dielectric constant real part ε ′ ≧ 350 when the measurement frequency is 1 MHz.   A capacitor using the ferroelectric ceramic obtained by the method according to claim 1, 2 or 3 or the ferroelectric ceramic according to claim 4.