JP7064204B2 - Lead-free ferroelectric composition and thin film - Google Patents

Description

The present invention relates to a lead-free ferroelectric composition and a thin film composed thereof.

Conventionally, as a composition exhibiting ferroelectricity (ferroelectric ceramics), a ferroelectric composition containing lead such as lead zirconate titanate (PZT) is widely known. Such a ferroelectric composition is excellent in properties as a ferroelectric substance (ferroelectricity) such as dielectric constant and piezoelectricity, and is widely used in various industrial fields.
However, in recent years, from the viewpoint of environmental protection, the movement to use lead-free substances and products has become widespread, and this is also the case in the field of ferroelectric compositions. Therefore, a lead-free ferroelectric composition, that is, a lead-free ferroelectric composition is required.
As the lead-free ferroelectric composition, for example, a composition containing bismuth titanate (BT), bismuth bismuth titanate (BLT) and the like is being studied (see, for example, Patent Document 1). However, even these lead-free ferroelectric compositions do not have sufficient polarization characteristics and have not completely replaced lead-containing ferroelectric compositions such as lead zirconate titanate.

Japanese Patent Application Laid-Open No. 2003-82470

In view of the above situation, it is an object of the present invention to provide a novel ferroelectric composition which does not contain lead and has excellent polarization performance, and a thin film composed thereof.

As a result of various studies by the present inventors in order to solve the above problems, a composition composed of an oxide containing Bi, Ti, Nd and Eu in a specific ratio exhibits excellent polarization performance. And came to the present invention.

That is, in the first invention, the element ratios of Bi, Ti, Nd and Eu to the total amount of Bi, Ti, Nd and Eu are 40 to 65 at%, 35 to 55 at%, 1 to 15 at%, and 1 The present invention relates to a lead-free ferroelectric composition comprising an oxide containing Bi, Ti, Nd and Eu in an amount of about 15 at%. The first invention may be a molded product made of the lead-free ferroelectric composition. Preferably, the relative density of the molded product is 40 to 60%. The first invention may be a sintered body made of the lead-free ferroelectric composition. Preferably, the relative density of the sintered body is 80% or more.

In the second invention, the element ratios of Bi, Ti, Nd and Eu to the total amount of Bi, Ti, Nd and Eu are 40 to 65 at%, 35 to 55 at%, 1 to 15 at%, and 1 to 1. The present invention relates to a lead-free ferroelectric oxide thin film comprising an oxide containing Bi, Ti, Nd and Eu, which is 15 at%. Preferably, the oxide thin film has crystal orientation in the substantially [200] direction. Further, preferably, the oxide thin film has a plate-like structure in the surface observation of the thin film by a field emission scanning electron microscope. The second invention may be a multiferroic element containing the lead-free ferroelectric oxide thin film, a piezoelectric element, a ferroelectric memory, or a sensor or an actuator.

According to the present invention, it is possible to provide a novel ferroelectric composition which does not contain lead and has excellent polarization performance, and a thin film composed thereof.

A photograph of the surface of the oxide thin film of Example 1 taken with a field emission scanning electron microscope. A photograph of a cross section of the oxide thin film of Example 1 taken with a field emission scanning electron microscope.

Hereinafter, embodiments of the present invention will be described in detail.
The ferroelectric composition according to the present invention is composed of an oxide containing bismuth (Bi), titanium (Ti), neodymium (Nd) and europium (Eu) without containing lead. The oxide may be a composite oxide, or may be a mixture of a simple oxide and / or a composite oxide.

In the oxide, the element ratios of Bi, Ti, Nd and Eu to the total amount of Bi, Ti, Nd and Eu are 40 to 65 at%, 35 to 55 at%, 1 to 15 at% and 1 to 15 at%. Is. When the ratio of the number of moles of each element is within the above range, the polarization performance exhibited by the ferroelectric composition can be improved. The element ratios of Bi, Ti, Nd and Eu are preferably 45 to 60 at%, 37 to 54 at%, 1 to 10 at%, and 1.1 to 10 at%, and more preferably 48 to 55 at%. , 37 to 52 at%, 1.1 to 8 at%, and 1.2 to 8 at%.

The lead-free ferroelectric composition may be a molded body formed to have a predetermined shape. The relative density of the molded product is, for example, preferably 40 to 60%, more preferably 42 to 55%, and even more preferably 44 to 50%.

Further, the lead-free ferroelectric composition may be a sintered body of an oxide. The relative density of the sintered body is preferably 80% or more and 100% or less, more preferably 85% or more, further preferably 90% or more, and particularly preferably 95% or more.

Further, the present invention may be a lead-free ferroelectric oxide thin film made of the oxide. Since the thin film exhibits good polarization performance, it is preferable that the thin film has crystal orientation in the substantially [200] direction in X-ray diffraction (XRD) measurement. Further, it is preferable that the thin film has a plate-like structure when the surface thereof is observed with a field emission scanning electron microscope. Here, the plate-shaped structure means that, as shown in FIGS. 1 and 2, many side surfaces of the plate-shaped crystal structure are observed on the surface of the thin film.

Next, a method for producing a ferroelectric composition, a molded body thereof, a sintered body, and a thin film according to the present invention will be specifically described, but the present invention is not limited to the following production methods. ..

(Raw material preparation process)
First, bismuth oxide or its precursor, titanium oxide or its precursor, neodymium oxide or its precursor, and europium oxide or its precursor are prepared as raw materials. As the precursor, a substance that can be converted into an oxide by a firing step or the like described later can be used, and specifically, a hydroxide, a carbonate, a nitrate or the like of each element can be used.

(Grinding and mixing process)
Next, each prepared raw material is crushed and mixed. Grinding and mixing can be performed using known methods. The mixing method may be dry mixing or wet mixing, but in the case of wet mixing, it is preferable to perform wet mixing in an alcohol solvent such as methanol or ethanol. As the device used for crushing and mixing, for example, various mixing devices such as a ball mill, a mortar, and a stirrer can be used. After performing the wet mixing, it may be appropriately dried.

(Baking process)
Next, the mixed raw material is fired to produce a lead-free ferroelectric composition. The firing temperature is preferably in the range of 500 ° C. to 1000 ° C., but the optimum temperature can be selected depending on the composition of the raw material.

The firing time is preferably in the range of 1 hour to 10 hours, but the optimum time can be selected depending on the composition of the raw material and the size of the raw material. The calcination can be performed under various conditions such as a normal atmosphere atmosphere, an atmosphere in which an inert gas and oxygen are mixed, or an oxygen atmosphere.

(Molding process)
Next, the powder of the lead-free ferroelectric composition obtained by the firing step is molded. Molding can be performed using various known molding equipment such as a die and a press machine. The pressure applied during molding is preferably in the range of, for example, 10 MPa to 400 MPa, but the optimum pressure can be selected depending on the composition of the composition and the size of the powder.

By selecting the pressure and time of this molding step, it is preferable that the relative density of the obtained molded product is 40 to 60%. The relative density of the molded product is more preferably 42 to 55%, still more preferably 44 to 50%.

(Sintering process)
When producing a sintered body, the above-mentioned raw materials can be mixed and the sintered body can be produced by hot pressing. In that case, each powdered raw material can be filled in a carbon die, and hot pressing can be performed under the conditions of a temperature of 500 to 1000 ° C. and a pressure of 10 to 200 MPa. Further, the sintered body can also be produced by producing the molded body as described above and then firing the molded body in an air atmosphere. In producing the sintered body, water and / or a binder (for example, polyvinyl alcohol, etc.) may be added to the raw material as a molding auxiliary material.

By selecting the temperature, pressure, and time of the sintering step, it is preferable that the relative density of the obtained sintered body is 80% or more and 100% or less. The relative density of the sintered body is more preferably 85% or more, further preferably 90% or more, and particularly preferably 95% or more.

(Manufacturing process of oxide thin film)
The oxide thin film composed of the lead-free ferroelectric composition of the present invention can be produced, for example, by a sputtering method. Specifically, the oxide thin film can be formed on a substrate by preparing a target made of a molded body or a sintered body of a lead-free ferroelectric composition and using a high-frequency magnetron sputtering device or the like. ..

As the material constituting the substrate, it is preferable that the lead-free ferroelectric oxide thin film of the present invention has a plate-like structure, for example, titanium oxide, zirconia oxide, aluminum oxide, or a fifth thereof. Examples thereof include those to which a group element is added. Further, a platinum film may be formed on the substrate, and the lead-free ferroelectric oxide thin film of the present invention may be formed on the platinum film.

The thickness of the lead-free ferroelectric oxide thin film of the present invention is not particularly limited, but is preferably in the range of 500 nm to 5000 nm.

If there is a possibility that bismuth may evaporate depending on the sputtering conditions or the manufacturing conditions of the sintered body, a desired amount of bismuth in the thin film is obtained with respect to the molded body or the sintered body for the purpose of supplementing the evaporating bismuth in advance. Bismuth oxide or a precursor thereof can be added in excess of the above.

As the sputtering conditions, for example, a mixed gas having an arbitrary mixing ratio of argon and oxygen is used as the atmospheric gas, the flow rate is 10 to 100 sccm, the substrate heating temperature is 550 ° C to 700 ° C, and the vacuum degree is 1.0 × 10 ⁻ . The film formation time can be set to ³ Pa or less, the film formation time can be set to 30 to 300 min, and the active power can be set to 100 to 200 W.

The lead-free ferroelectric oxide thin film of the present invention is not limited to that produced by the above method. Any oxide thin film finally obtained may have a predetermined composition. As a method for producing a lead-free strong dielectric oxide thin film other than the above-mentioned method, for example, a sol-gel method, a coating method, etc. Organic Metal Chemical Vapor Deposition (MOCVD), Physical Vapor Deposition (PVD), or Chemical Vapor Deposition (CVD) to form a thin film, or after firing a molded body, a high frequency magnetron sputtering device. A method of forming a thin film using the above can be mentioned.

Specific applications to which the lead-free ferroelectric oxide thin film of the present invention is applied are not particularly limited, and examples thereof include multiferroic elements, piezoelectric elements, ferroelectric memories, sensors, actuators, and the like. ..

Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.
(Examples 1 and 2 and Comparative Examples 1 to 3)
Raw material preparation process As raw materials, powdered bismuth oxide (purity 99.9% or more, particle size 2.6 μm), powdered titanium oxide (purity 99.9% or more, particle size 3.1 μm), powdered neodymium oxide (Purity 99.9% or more, particle size 1.6 μm) and powdered europium oxide (purity 99.9% or more, particle size 1.8 μm) were prepared. Each raw material was adjusted and weighed so that the element ratio finally obtained was the element ratio shown in Table 1.

Grinding and mixing steps Next, the prepared powders of each raw material were crushed and mixed. The pulverization and mixing were carried out by wet mixing with ethanol using a mortar. Then, it was dried at 90 ° C. for 2 hours using a dryer (Labostar GRAVITY OVEN LG-122 type).

Firing Step Next, the mixed raw materials were calcined to produce a lead-free ferroelectric composition. The firing was carried out in an air atmosphere using an electric furnace (MPH-1N type manufactured by AS ONE) at 750 ° C. for 3 hours. The temperature rising rate was 5 ° C./min and the cooling rate was 4 ° C./min. After firing, it was cooled to room temperature to obtain a powdery lead-free ferroelectric composition.

Molding Step Next, a lead-free ferroelectric composition was molded. Molding was carried out by using a mold having a diameter of 100 mm and maintaining a pressure of 40 MPa for 2 minutes. As a result, a cylindrical molded body having a diameter of 100 mm and a thickness of about 5 mm and having a relative density of 48% composed of a lead-free ferroelectric composition was obtained.

Film formation step As a substrate for film formation, a TiO ₂ single crystal substrate doped with 1.0 mass% Nb was used. The size of the substrate is 7.0 × 7.0 mm and the thickness is 0.4 mm.

Using a high-frequency magnetron sputtering device (SPF-210B manufactured by Anerva Co., Ltd.) as a target for a molded body composed of the lead-free ferroelectric composition obtained in the molding step, the film thickness 1 is placed on the TiO ₂ substrate. An oxide thin film composed of a 0.8 μm lead-free ferroelectric composition was formed. Sputtering was performed under the following conditions.
・ Atmosphere: Ar / O ₂ = 9: 1 (50sccm)
-Substrate temperature: 650 ° C
・ Vacuum degree: 5.0 × 10 ^-4 Pa
・ Film formation time: 200 min
・ Active power: 120W

Using a high-frequency multilayer sputtering device (manufactured by Anerva Co., Ltd.), a Ti electrode is prepared on the back surface of the TiO ₂ substrate, and a Pt electrode is further formed on the upper surface of an oxide thin film composed of the lead-free ferroelectric composition. After preparation, the characteristics of the oxide thin film composed of the lead-free ferroelectric composition were evaluated.

(Evaluation methods)
(FE-SEM observation and TEM observation)
The surface of the oxide thin film was observed under the conditions of an acceleration voltage of 5 kV and a WD of 10 mm using a field emission scanning electron microscope (JSM-7001F manufactured by JEOL), and the confirmed structure is shown in Table 1. FIG. 1 shows a micrograph of the surface of the oxide thin film of Example 1 as described above.
Further, FIG. 2 shows a photomicrograph of a cross section of the oxide thin film taken under the condition of an acceleration voltage of 200 kV using a transmission electron microscope (JEM2100 manufactured by JEOL).

(Measuring method of element ratio of oxide thin film)
Using the energy dispersive X-ray spectroscope attached to the electric field radiation scanning electron microscope (JEOL JSM-7001F), adjust the irradiation current so that the count count is 8000 cps or more under the conditions of acceleration voltage of 15 kV and magnification of 500 times. And the measurement was performed. Table 1 shows the average value of the five values obtained by five-field photography.

(Measurement method of polarization)
A polarization-electric field (PE) hysteresis loop was performed using a binocular system equipped with a manipulator (Kyowa Riken Microfiber K-160MP, Nikon binoculars) and a ferroelectric measuring device (Radiant Tech. RT-66A). It was measured. The applied electric field was 500 kV / cm. The polarization value obtained when scanning with the above applied electric field-the polarization value when the applied electric field is 0 kV / cm in the applied electric field hysteresis loop is defined as the residual polarization _Pr , and the polarization value can be ±, so it is doubled. Is shown in Table 1 as the polarization performance.

(Measurement method of film thickness)
Using a transmission electron microscope (JEM2100 manufactured by JEOL), a transmission image of a sample cross section was photographed at an acceleration voltage of 200 kV, and the film thickness was measured.

(Measuring method of crystal orientation)
The crystal orientation was measured using an X-ray diffractometer (D8μ-HR manufactured by Bruker) equipped with a two-dimensional detector, and the results are shown in Table 1. The measurement conditions were a tube voltage of 40 kV, a tube current of 40 mV, a step time of 200 s / step, and 2θ: 20-70 °.

From Table 1, it can be seen that Examples 1 and 2 are excellent in polarization performance. On the other hand, Comparative Example 1 was a sample to which Europium was not added, and showed low polarization performance as compared with Examples 1 and 2. Further, Comparative Example 2 was a sample to which neodymium was not added, and Comparative Example 3 was a sample to which both europium and neodymium were not added, both of which showed significantly lower polarization performance as compared with Examples 1 and 2. ..

Claims (12)

The element ratios of Bi, Ti, Nd and Eu to the total amount of Bi, Ti, Nd and Eu are 40 to 65 at%, 35 to 55 at%, 1 to 10 at% , and 1.1 to 10 at% . A lead-free ferroelectric composition comprising an oxide containing Bi, Ti, Nd and Eu. A molded product made of the lead-free ferroelectric composition according to claim 1. The molded product according to claim 2, wherein the relative density of the molded product is 40 to 60%. A sintered body made of the lead-free ferroelectric composition according to any one of claims 1 to 3. The sintered body according to claim 4, wherein the relative density of the sintered body is 80% or more. The element ratios of Bi, Ti, Nd and Eu to the total amount of Bi, Ti, Nd and Eu are 40 to 65 at%, 35 to 55 at%, 1 to 10 at% , and 1.1 to 10 at% . A lead-free ferroelectric oxide thin film comprising an oxide containing Bi, Ti, Nd and Eu. The non-lead ferroelectric oxide thin film according to claim 6, wherein the oxide thin film has crystal orientation in the substantially [200] direction. The non-lead ferroelectric oxide thin film according to claim 6 or 7, wherein the oxide thin film has a plate-like structure in surface observation of the thin film by an electric field radiation scanning electron microscope. A multiferroic device comprising the lead-free ferroelectric oxide thin film according to any one of claims 6 to 8. A piezoelectric element containing the lead-free ferroelectric oxide thin film according to any one of claims 6 to 8. A ferroelectric memory comprising the lead-free ferroelectric oxide thin film according to any one of claims 6 to 8. The sensor or actuator comprising the lead-free ferroelectric oxide thin film according to any one of claims 6 to 8.

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