JP2004210601A - Dielectric material having high dielectric constant, and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a BaTi<SB>2</SB>O<SB>5</SB>single crystal exhibiting an extremely high dielectric constant, and to provide its manufacturing method. <P>SOLUTION: The dielectric material exhibiting the high dielectric constant is a plate-like piece of the BaTi<SB>2</SB>O<SB>5</SB>single crystal. The surface parallel to the electrode surface is (010)-plane of the crystal. The dielectric material is manufactured by mixing powdery raw materials so that the atomic concentration ratio of Ti to Ba is in a range of 1.8 to 2.2, then sintering the resulting mixture into a solidified body, preparing the BaTi<SB>2</SB>O<SB>5</SB>single crystal by melting and solidifying the solidified body with a solidification speed of 5-50 mm/h by a floating zone melting method, and cutting out cut pieces from the crystal so that the surface parallel to the (010)-plane of the crystal becomes the electrode surface. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
[Technical field to which the invention belongs]
The present invention relates to a dielectric having a high dielectric constant made of a single crystal of BaTi ₂ O ₅ , a method for producing a single crystal used as a material for the dielectric, and a method for producing the dielectric.
[0002]
[Prior art]
Since the discovery of barium titanate (BaTiO ₃ ), a composition having a 1: 1 ratio of BaO to TiO ₂ that exhibits a high dielectric constant, BaTiO ₃ -based ceramic compositions have been put into practical use as high dielectric constant capacitor materials. Various improvements and developments have been made.
[0003]
Among the BaO-TiO ₂ ceramics represented by the BaTiO ₃ is other like BaTi ₄ O ₉ or _Ba 2 _Ti 9 _{O 20} was studied as a microwave band, a high dielectric constant of greater than BaTiO ₃ system The composition has not yet been found.
[0004]
In addition, many studies have been conducted on the phase diagram of BaO—TiO ₂ which is the basis of this type of ceramics. However, there is an unstable phase and it is not yet the final established state diagram. One of the unstable ones is a composition having a composition ratio of BaO to TiO ₂ of 1: 2 between BaTiO ₃ and Ba ₆ Ti ₁₇ O ₄₀ , that is, BaTi ₂ O ₅ .
[0005]
BaTi ₂ O ₅ is considered a metastable phase, but there is a report that it is a stable low-temperature phase. Normally, when the oxide atomic concentration composition ratio Ti / Ba is 2.0 and it is melted at high temperature and then cooled, it appears mixed with other phases and cannot be obtained as a single phase. do not do. However, a single powder was obtained by hydrolysis at a low temperature of 1073 K, and the crystal structure and lattice constant were measured, but little more is known about the physical properties beyond that.
[0006]
[Problems to be solved by the invention]
An object of the present invention, a dielectric consisting of BaTi ₂ O ₅ single crystal showing a very high dielectric constant, a method of manufacturing thereof becomes becomes a dielectric material BaTi ₂ O ₅ single crystal, and a production method of the dielectric There is to do.
[0007]
[Means for Solving the Problems]
The present inventors relates to ceramic BaO-TiO ₂ system, to clarify the physical properties thereof were subjected to various research studies to improve the dielectric performance. At that time, in order to investigate the physical properties of each phase appearing in the BaO-TiO ₂ system, particularly in the region where TiO ₂ exceeds 50 mol%, an attempt was made to produce a single-phase single crystal. It was found that when Ti is 1: 2, a single crystal of BaTi ₂ O ₅ can be produced by using the floating zone melting method.
[0008]
The crystal structure of BaTi ₂ O ₅ is a bottom-centered monoclinic lattice at room temperature, but when measuring the dielectric constant with the obtained single crystal, plate-like test pieces are cut out parallel to various surfaces having a low surface index, As a result of measuring the dielectric constant, it was clear that when a potential was applied in a direction perpendicular to the (010) plane, an extremely high dielectric constant was exhibited.
[0009]
In the case of BaTiO ₃ , there is a peak indicating the maximum dielectric constant in the vicinity of 390 K of the Curie temperature causing the phase transition, and the value is 6000 to 10,000. As a practical capacitor material, various additives are used to shift the temperature at which the dielectric constant reaches a peak to near room temperature, widen the peak, increase the dielectric constant at room temperature, and temperature dependence. Trying to make it smaller. However, since the ferroelectricity is lost due to phase transformation and changes to a paraelectric material above the peak temperature, it is difficult to apply it to a ferroelectric memory or the like. Therefore, a material having a high dielectric constant with a high Curie temperature is required.
[0010]
When obtained BaTi ₂ O ₅ Examining the dielectric constant of the single crystal, exceeds 25000 peaked near 750K in a direction perpendicular to the (010) plane, a dielectric constant is 100kHz at that time. This value is much higher than the peak value of BaTiO ₃ and the Curie temperature is high, so that the ferroelectricity is maintained up to a high temperature. As in the case of BaTiO ₃ , such characteristics have the possibility of lowering the peak temperature and reducing temperature dependence by utilizing various additives, and various developments are expected as dielectrics for capacitors.
[0011]
The reason why the single phase single crystal of BaTi ₂ O ₅ can be stably obtained by using the floating zone melting method is not necessarily clear, but for one thing, the floating zone melting method is a seed crystal. Since the crystal is grown by unidirectional solidification in which the melt is solidified on it, it cannot be separated into other phases after solidification, and it may be left as it is. Because the seed crystals did not have a single crystal of BaTi ₂ O _5, Ba: atomic weight ratio of Ti is 1: was used 2 of melting and solidification crystals, single crystals in the course of initial solidification is produced, it directly grown Probably it was.
[0012]
The other is that the floating zone melting method does not come into contact with other objects such as crucibles, so the absence of impurities during the melting process may have facilitated single-phase crystal formation.
[0013]
Thus, the dielectric constant of the peak value are generated in the direction perpendicular to the (010) plane of the single crystal, the conventional BaTiO ₃ system showed a high value not obtained, the material in such a condenser If a higher dielectric constant is required, it may be effectively applied. That is, the gist of the present invention is as follows.
[0014]
(1) A high-dielectric-constant dielectric, characterized in that it is a BaTi ₂ O ₅ single crystal plate-like piece, and the plane parallel to the electrode surface is the (010) plane of the crystal.
[0015]
(2) The raw material powder is mixed with the Ti / Ba atomic concentration ratio (Ti / Ba) in the range of 1.8 to 2.2 and sintered to form a solid, and then solidified by the floating zone melting method. A method for producing a single crystal of BaTi ₂ O ₅ , wherein melting and solidification are performed at ₅ to 50 mm / h.
[0016]
(3) The above (1), wherein the single crystal produced by the method described in (2) above is used, and a piece is cut out so that a plane parallel to the (010) plane of the crystal becomes an electrode surface. A method for producing a high dielectric constant dielectric.
[0017]
DETAILED DESCRIPTION OF THE INVENTION
The dielectric of the present invention is a single crystal plate-like piece of BaTi ₂ O ₅ , wherein a plane parallel to the electrode surface is a (010) plane of the crystal. A single crystal of BaTi ₂ O ₅ exhibits a very high dielectric constant when a potential is applied in a direction perpendicular to the (010) plane of the crystal, but does not exhibit a high dielectric constant when applied in other directions. .
[0018]
When a plate-like dielectric used for a capacitor or the like is a ceramic single crystal, it is difficult to directly produce a plate-like single crystal. Therefore, after producing a single crystal and confirming the crystal orientation of the single crystal, ( The plate-shaped piece may be cut out so that the (010) plane is parallel to the electrode plane and the potential is applied perpendicularly to the (010) plane.
[0019]
For manufacturing the single crystal, a floating zone melting method used for manufacturing a high-purity silicon single crystal for a substrate of a semiconductor integrated circuit is used. Raw material is a composition containing atomic weight ratio Ti / Ba of Ti and Ba is 1.8 to 2.2, and finally if the takes the form of BaO · 2TiO _2, for example, rutile and anatase powder BaCO ₃ powder or the like may be used. When Ti / Ba is set to 1.8 to 2.2, the amount of BaTiO ₃ is increased in BaTi ₂ O ₅ when it is less than 1.8, and Ba ₆ Ti ₁₇ O ₄₀ is exceeded when it exceeds 2.2. This is because, in any case, the dielectric constant decreases.
[0020]
Since the raw material melts in the floating zone, a rod-shaped material is preferably formed in advance by a method of sintering powder. In this case, for example, the raw material powder is kneaded, and if necessary, a binder is added and pressure-molded into a required shape and fired in the atmosphere at a heating temperature of 1200 to 1600K.
[0021]
In the floating zone melting method, the sintered rod of the material is arranged vertically, heated and melted from the lower end and contact fused to the seed crystal, and then the lower part is solidified while moving the melting part gradually toward the upper end Single crystals are grown, and the solidification growth rate at that time is preferably 5 to 50 mm / h. This depends on the diameter of the single crystal, but at a growth rate of less than 5 mm / h, it is easy for the melt to fall from the melted part, and at a rate exceeding 50 mm / h, polycrystallization occurs and a single crystal is obtained. It is because there is a risk of disappearing.
[0022]
After the single crystal is obtained, a seed crystal having a preferred orientation in the crystal growth direction may be cut out and used. However, using a sintered body or a molten solid body having a Ti / Ba of 1.8 to 2.2. Also good.
[0023]
【Example】
BaCO ₃ and TiO ₂ powders each having a purity of 99.9% were mixed so that the atomic concentration ratio Ti / Ba was 2.0, and a small amount of ethanol was added and kneaded in a mortar. The kneaded material was pressed into a rod shape having a diameter of about 8 mm and molded at 10 MPa, and sintered in air at 1500 K for 12 hours.
[0024]
In a floating zone melting apparatus using a sintered rod as a raw material and a xenon lamp as a heat source, the seed crystal is a sintered body having the same composition, oxygen is 21% by volume, and the balance is 20 mm / h in an argon stream. Unidirectional solidification was performed at the solidification rate.
[0025]
The obtained crystal was a single crystal having a diameter of about 5 mm and no colorless and transparent grain boundaries. The powder was prepared and the phase was identified by X-ray diffraction, and the crystal orientation of the single crystal was examined by the X-ray pole diffraction method (Laue method). A plate-shaped test piece having a width of 2 mm, a length of 3 mm, and a thickness of 1 mm was cut out in parallel to the (100) plane, (010) plane, or (001) plane of the single crystal, and the dielectric constant in the direction perpendicular to the plane of the plate was measured. Measurement was performed at a frequency of 100 kHz using an AC impedance analyzer (Solartron 1260, 1296). The measurement was performed in the atmosphere, and the temperature range was 293-1073K.
[0026]
For comparison, a kneaded material having an atomic concentration ratio Ti / Ba of 2.0, and a kneaded material having an atomic concentration ratio Ti / Ba of 1.0 using the same raw material powder, a diameter of 10 mm, a thickness A 3 mm disk was pressed and molded at 10 MPa perpendicular to the surface, and each disk was sintered in the atmosphere by firing at 1500 K for 12 hours. For these sintered specimens, the change in dielectric constant with temperature at 293 to 1073 K was measured in the same manner as described above.
[0027]
FIG. 1 and FIG. 2 show the powder X-ray diffraction patterns of the crystal after floating zone melting and the sintered body after firing when Ti / Ba is 2.0, respectively. The floating zone after melt crystallization as shown in Figure 1, all of the peaks are indexed as BaTi ₂ O _5, it can be seen that BaTi ₂ O ₅ single phase is obtained. In contrast, in the sintered body of the same composition of Figure 2, serves as the secondary phase of BaTiO ₃ and _Ba 6 _Ti 17 _{O 40} are mixed, the generation of BaTi ₂ O ₅ is not observed.
[0028]
The dielectric constant of the single crystal increased sharply from 700K and showed a maximum value of 27000 at 748K when measured by applying a potential perpendicular to the surface of the sample cut out parallel to the (010) plane of the crystal. On the other hand, the dielectric constant was about 140 for the sample parallel to the (100) plane and about 70 for the sample parallel to the (001) plane, and there was almost no change with temperature in either case.
[0029]
FIG. 3 shows a sample cut out parallel to the (010) plane of the single crystal, a sample cut out parallel to the (001) plane, a sintered body when Ti / Ba is 2.0, and a Ti / Ba of 1.0. for BaTiO ₃ sintered body, showing the measurement results of the change in dielectric constant in 293～1073K. As can be seen, the BaTiO ₃ sintered body has a peak dielectric constant of 7000 at 393 K, whereas it is 27000 at 748 K in the direction perpendicular to the (010) plane of the BaTi ₂ O ₅ single crystal of the dielectric of the present invention. 3 times higher value is shown.
[0030]
Further, in the sintered body with Ti / Ba of 2.0 (composition ratio BaTi ₂ O ₅ ), there is a dielectric constant peak at 393 K, which is about 3000, and the BaTiO ₃ phase in the sintered body The dielectric constant is considered.
[0031]
【The invention's effect】
The dielectric of the present invention exhibits a dielectric constant far higher than BaTiO ₃ which is conventionally known to exhibit the highest dielectric constant among BaO—TiO ₂ ceramics. Method of moving the temperature indicating this high dielectric constant or temperature-dependent improvement, is for further study, since maximum dielectric constant attainable is much higher than BaTiO _3, a new high dielectric constant capacitor materials Be expected.
[0032]
In addition, since this dielectric can be easily manufactured by applying the floating zone melting method that has already been put into practical use with silicon single crystals and the like, it is easy to further develop the characteristics of BaTi ₂ O ₅ -based materials and practical production. It is.
[Brief description of the drawings]
FIG. 1 is a powder X-ray diffraction pattern of BaTi ₂ O ₅ produced by a floating zone melting method.
FIG. 2 is a powder X-ray diffraction pattern of BaTi ₂ O ₅ produced by a sintering method.
FIG. 3 is a measurement result of a dielectric constant (measured at 100 kHz) of a prototype dielectric measured in a temperature range of 293 to 1073K.

Claims (3)

A plate-like piece of single crystals of BaTi ₂ O _5, high-k dielectric to the electrode surface and the plane parallel is characterized in that it is a (010) plane of the crystal. The raw material powder is mixed so that the atomic concentration ratio of Ti and Ba (Ti / Ba) is in the range of 1.8 to 2.2 and sintered to form a solid. A method for producing a single crystal of BaTi ₂ O ₅ , wherein melting and solidification are performed at 50 mm / h. 2. The high dielectric according to claim 1, wherein the dielectric is cut out using a single crystal produced by the method according to claim 2 so that a plane parallel to the (010) plane of the crystal becomes an electrode surface. A method for manufacturing a dielectric constant.

Images