JP5170580B2 - Method for producing bismuth layered structure ferroelectric crystal - Google Patents

Description

  The present invention relates to a method for producing a ferroelectric crystal having a bismuth layer structure.

  Bismuth layered compounds have been studied as materials for ferroelectric memories and piezoelectric elements. In order to obtain high characteristics in these elements, it is necessary to reduce the coercive electric field Ec of the bismuth layered compound and increase the remanent polarization Pr.

  Various values have conventionally been reported for the coercive electric field Ec and remanent polarization Pr of the bismuth layered compound (for example, Non-Patent Documents 1 to 3).

  On the other hand, various attempts have been made to improve the characteristics of the bismuth layered compound. For example, methods for heat-treating the bismuth layered compound in an atmosphere containing oxygen have been proposed (for example, Patent Document 1 and Non-Patent Documents). Reference 2). This Patent Document 1 describes that the deterioration of piezoelectric characteristics due to the pyroelectric effect can be reduced by heat treatment.

JP 2002-284573 A

“Key Engineering Materials”, 2002, Vols. 228-229, pp 223-226 “Japanese Journal of Applied Physics”, 2002, Vol. 41, pp. L1164-1166 “Journal of Applied Physics”, USA, 2001, Vol. 90, no. 8, p4089-4091

  However, it has been difficult for the conventional bismuth layered compound to achieve both a small coercive electric field Ec and a large remanent polarization Pr.

  In view of such circumstances, an object of the present invention is to provide a bismuth layer-structure ferroelectric crystal having a small coercive electric field Ec and a large remanent polarization Pr, a method for producing the same, and an electronic device using the same.

  As a result of investigations to achieve the above object, the present inventors have made it possible to significantly reduce the coercive electric field Ec and the residual polarization Pr by heat-treating a high-quality bismuth layer-structure ferroelectric crystal under a high oxygen partial pressure. I found the phenomenon to become large for the first time. The present invention is based on this new finding. The values of remanent polarization Pr and coercive electric field Ec described below are values when the hysteresis characteristics are measured at 25 ° C.

The bismuth layered structure ferroelectric crystal of the present invention is a layered structure ferroelectric crystal containing bismuth and has a residual polarization Pr (μC / cm ² ) and a coercive electric field Ec (when a hysteresis characteristic is measured at 25 ° C. kV / cm) satisfies Ec ≦ 35 and 1 <Pr / Ec.

  In addition, the method of the present invention for producing a bismuth layered structure ferroelectric crystal includes a layered structure ferroelectric crystal containing bismuth using a material containing at least one bismuth supply material selected from bismuth and a bismuth compound. The purity of bismuth contained in the at least one bismuth supply material includes a step of forming (A) and a heat treatment step of heat-treating the crystal (A) in an atmosphere having an oxygen partial pressure of 1 atm or higher. It is 99.999 mol% or more. That is, the ratio of impurity metal elements other than bismuth among the metal elements contained in the bismuth supply material is less than 0.001 mol%. Hereinafter, in the material for supplying the specific metal element constituting the crystal, the molar ratio of the specific metal element to the entire metal element contained in the material may be referred to as “metal molar ratio” (hereinafter, referred to as “metal molar ratio”). The elements L and R to be described are all considered as metal elements). The crystal produced by this method is another bismuth layered structure ferroelectric crystal of the present invention.

  The electronic device of the present invention uses the bismuth layered structure ferroelectric crystal of the present invention.

  According to the present invention, it is possible to obtain a bismuth layered structure ferroelectric crystal that can achieve both a small coercive electric field Ec and a large residual polarization Pr as compared with a conventional bismuth layered structure ferroelectric. According to the present invention, even when lead is not included, a ferroelectric having a small coercive electric field Ec and a large remanent polarization Pr can be obtained. By using such a ferroelectric of the present invention, an electronic device having high characteristics can be obtained.

It is a figure which shows an example of the hysteresis characteristic of the ferroelectric crystal of this invention and a comparative example. It is a figure which shows an example of the hysteresis characteristic of the ferroelectric crystal of this invention. It is a figure which shows an example of the leakage current of the ferroelectric crystal of this invention, and the ferroelectric crystal of a comparative example.

  Embodiments of the present invention will be described below. First, the method of the present invention for producing a bismuth layered structure ferroelectric crystal will be described.

  The production method of the present invention includes a step of forming a layered structure ferroelectric crystal (A) containing bismuth using a material containing at least one bismuth supply material selected from bismuth and a bismuth compound. At this time, it is important for obtaining a high effect that the purity of bismuth contained in the bismuth supply material is 99.999 mol% or more. Examples of the bismuth compound include bismuth oxide, bismuth nitrate, and bismuth alkoxide.

  The crystal (A) is a bismuth layered structure ferroelectric crystal having a structure in which bismuth oxide layers and perovskite type structural blocks are alternately stacked. The perovskite structure block includes at least one element L selected from Li, Na, K, Ca, Sr, Ba, Y, Bi, Pb and rare earth elements, Ti, Zr, Hf, V, Nb, Ta, It is composed of at least one element R selected from W, Mo, Mn, Fe, Si and Ge and oxygen.

Typically, the composition of the crystal (A) is represented by the following general formula. This crystal (A) has a structure in which bismuth oxide layers [(Bi ₂ O ₂ ) ²⁺ ] and perovskite-type structural blocks [(L _m-1 R _m O _{3m + 1} ) ^2- ] are alternately stacked.

(Bi ₂ O ₂ ) (L _m-1 R _m O _{3m + 1} )
[Wherein L represents at least one element selected from Li, Na, K, Ca, Sr, Ba, Y, Bi, Pb and rare earth elements. R represents at least one element selected from Ti, Zr, Hf, V, Nb, Ta, W, Mo, Mn, Fe, Si, and Ge. m represents an integer of 1 to 5. ]
Examples of rare earth elements that can be used as the element L include La, Ce, Pr, Nd, and Sm. From the viewpoint of environment, it is preferable that the crystal (A) does not contain lead (Pb). The material for forming the crystal (A) is selected according to the composition of the crystal (A).

The types and amounts of the elements L and R are preferably selected so that the charge of the entire crystal (A) becomes zero. For example, when Bi ³⁺ is used as the element L and Ti ⁴⁺ is used as the element R, m is 3. An element that breaks the balance of charge may function as an impurity, and the influence is particularly great when the crystal (A) is a single crystal. For example, in a Bi ₄ Ti ₃ O ₁₂ single crystal, a divalent metal ion may function as an impurity. For this reason, it is preferable that the amount of the element that breaks the balance of electric charges is as small as possible. The amount of such elements contained in the material for producing the crystal (A) is preferably less than 0.01% (metal molar ratio), and less than 0.001% (metal molar ratio). Is more preferable.

  The form of the crystal (A) is not particularly limited, and may be a bulk or a thin film. Further, the crystallinity of the crystal (A) is not limited, and may be any of single crystal, polycrystal, or microcrystal. When the crystal (A) is a single crystal, a bismuth layer structure ferroelectric single crystal exhibiting a particularly small coercive electric field and a large remanent polarization can be obtained by a heat treatment step described later.

A preferred example of the crystal (A) is a crystal (particularly a single crystal) having a basic composition of Bi ₄ Ti ₃ O ₁₂ . Here, the basic composition refers to a composition in which the content of elements other than Bi, Ti, and O is less than 5 atomic%. _Further, Bi ₄ Ti _{3 O 12} in a part of Ti was replaced with the element R (e.g., V, W and Nb) crystal (particularly a single crystal), or _Bi 4 _Ti 3 part of the above elements of _{O 12} and Bi A crystal (particularly a single crystal) substituted with L (for example, a rare earth element such as La or Nd), or a crystal (particularly a single crystal) in which Bi and part of Bi ₄ Ti ₃ O ₁₂ are simultaneously substituted with the elements L and R Is also preferred.

  In the method of the present invention, it is particularly important that the quality of the crystal (A) is high. The production method of the crystal (A) may be any method as long as it is a method capable of producing a high quality crystal (particularly a single crystal). For example, a method of melting and cooling a material, a chemical vapor deposition method (CVD method), a sputtering method, a pulse laser deposition method (PLD method), or the like can be applied. It is also considered that a method using a solution process such as a sol-gel method or a CSD (Chemical Solution Deposition) method can be applied. Hereinafter, an example of a method for producing a single crystal by melting the material and then cooling it will be described.

In this method, first, Bi ₂ O ₃ powder and powder of other materials (for example, TiO ₂ powder) are mixed. At this time, it is important that the purity of the material is high. The purity of the Bi ₂ O ₃ powder is preferably 99.999% (metal molar ratio) or more. The purity of the TiO ₂ powder is preferably 99.99% (metal molar ratio) or more.

Next, the mixed powder is melted by heating at a high temperature (for example, a range of 1050 ° C. to 1200 ° C.) for a certain time (for example, a range of 1 hour to 50 hours). Thereafter, it is gradually cooled to room temperature. In order to obtain a high-quality single crystal, the cooling rate when cooling to 1000 ° C. is preferably −5 ° C./hour or slower, for example, in the range of −5 ° C./hour to −1 ° C./hour. It can be. The cooling rate when cooling from 1000 ° C. to room temperature is preferably −5 ° C./hour or slower, but it may be faster, that is, it may be more negative than −5 ° C./hour. However, it is preferable to slowly cool as much as possible. The atmosphere during heating and cooling is not particularly limited, and may be, for example, an air atmosphere, but preferably contains at least oxygen. In this way, crystal (A) (for example, Bi ₄ Ti ₃ O ₁₂ single crystal) is obtained. According to this crystal manufacturing method, when the hysteresis characteristic is measured by applying an electric field in the a-axis or b-axis direction, the residual polarization is in the range of about ²⁰ to 40 μC / cm ² , and the coercive electric field is 40 to 100 kV / Crystals in the range of about cm can be produced.

  Next, the crystal (A) is heat-treated in an atmosphere having an oxygen partial pressure of 1 atm or higher (heat treatment step). Although this heat treatment process is considered to reduce a very small amount of oxygen defects present in the crystal (A), the composition of the crystal (A) is not substantially changed. Therefore, the crystal produced by the production method of the present invention, that is, the bismuth layered structure ferroelectric crystal of the present invention has the composition and crystal structure described for the crystal (A). For example, the form of the bismuth layered structure ferroelectric crystal of the present invention is not particularly limited, and may be a bulk or a thin film. The crystallinity of the bismuth layer structure ferroelectric crystal of the present invention is not limited, and may be any of single crystal, polycrystal, or microcrystal. Note that the crystallinity of the crystal (A) may be improved by the heat treatment.

  The oxygen partial pressure in the heat treatment step is preferably higher than 1 atmosphere, more preferably 10 atmospheres or more, and further preferably 50 atmospheres or more (for example, 100 atmospheres or more or 200 atmospheres or more). Although there is no particular upper limit on the oxygen partial pressure, it is usually set to 1000 atm or less. The atmosphere gas at the time of performing the heat treatment step may be only oxygen gas or may contain gas other than oxygen gas. Such heat treatment can be performed using a general pressurizing apparatus.

The heat treatment step is preferably performed at a temperature of 300 ° C. or higher and lower than the melting point of the crystal (A), for example, a temperature in the range of 300 ° C. to 1000 ° C. (preferably about 300 ° C. to 800 ° C.). The heat treatment step preferably includes a step of heating at least the phase transition temperature _{(Bi 4} Ti ₃ In _{O 12} to about 670 ° C.) above the temperature of the crystal (A). It is considered that oxygen is easily taken into the crystal (A) by heating to a temperature equal to or higher than the phase transition temperature. The heated crystal is cooled in an atmosphere having an oxygen partial pressure of 1 atm or higher (typically in the same atmosphere as during heating). In the method of the present invention, the rate of cooling the heated crystals is important. When cooling a crystal heated to a temperature higher than 300 ° C. (preferably higher than the phase transition temperature) to 300 ° C., a cooling rate of −5 ° C./min or slower (preferably less than −1 ° C./min) It is preferable to cool at a slow cooling rate, for example, a cooling rate slower than −0.5 ° C./min). The cooling rate when cooling from 300 ° C. to near room temperature may be faster, for example, −20 ° C./min or slower cooling rate (preferably at a rate slower than −5 ° C./min, for example − (Cooling rate slower than 2 ° C./min). The preferable heat treatment time varies depending on the heat treatment temperature and the cooling rate.

  In a typical example of the heat treatment step, first, the temperature of the crystal (A) is raised from room temperature to about 750 ° C. in a time of about 1 to 20 hours. It may be necessary to heat very slowly at a rate of about 1 ° C./hour near the phase transition temperature of the heating process. Thereafter, the temperature ± 10 ° C. is maintained for about 2 to 20 hours. Then, cooling is performed at a temperature decreasing rate of about -1 ° C / min to -5 ° C / min or a temperature decreasing rate lower than 400 ° C to 300 ° C. Then, it cools to room temperature with the temperature-fall rate of about -5 degrees C / min. A typical oxygen partial pressure in such a heat treatment step is 10 atmospheres or more, for example, in the range of 10 atmospheres to 200 atmospheres.

The characteristics of the crystal (A) can be greatly improved by the heat treatment step. In particular, the coercive electric field Ec can be reduced and the residual polarization Pr can be increased by the heat treatment process. The crystal obtained by this method exhibits a particularly small coercive electric field Ec and a particularly large remanent polarization Pr when the hysteresis characteristic is measured by applying an electric field in the a-axis or b-axis direction. For example, according to this method, when the hysteresis characteristic is measured by applying an electric field in the a-axis or b-axis direction of the crystal, the remanent polarization is 45 μC / cm ² or more and the coercive electric field is 35 kV / cm or less. It is possible to obtain a structural ferroelectric crystal. These values are values that were not achieved at all in the bismuth layered structure ferroelectrics. Also, by optimizing the conditions in the above method, it is possible to obtain a bismuth layered structure ferroelectric crystal having a remanent polarization of 50 μC / cm ² or more and a coercive electric field of 20 kV / cm or less.

Further, in the bismuth layered structure ferroelectric crystal of the present invention obtained by the above method, the hysteresis curve when the hysteresis characteristic is measured by applying an electric field in the a-axis or b-axis direction is a conventional bismuth layered structure ferroelectric. Compared to the hysteresis characteristics of crystals, it has a more rectangular shape. For example, in the bismuth layered structure ferroelectric crystal of the present invention, the residual polarization Pr (μC / cm ² ) and coercive electric field Ec (kV / cm ² ) when the hysteresis characteristic is measured by applying an electric field in the a-axis or b-axis direction. ), That is, the value of Pr / Ec is large. Specifically, Ec ≦ 35 and 1 <Pr / Ec can be achieved. Similarly, 45 ≦ Pr and 1 <Pr / Ec can be achieved.

The remanent polarization is the magnitude of polarization when the electric field is inverted, that is, when the applied electric field is 0 kV / cm. In FIG. 1, the magnitude of polarization at the intersection of the hysteresis curve and the Y axis. That's it. The coercive electric field is the magnitude of the electric field when the polarization is reversed, that is, when the polarization is 0 C / cm ² , and in FIG. 1, the magnitude of the electric field at the intersection of the hysteresis curve and the X axis. is there.

  The bismuth layered structure ferroelectric can be used in various fields, and in particular, a high effect can be obtained by using it in an electronic device. Typical electronic devices to which the ferroelectric crystal of the present invention can be applied include a nonvolatile memory and a piezoelectric device. These can be realized by replacing the ferroelectric portion of a known electronic device with the ferroelectric crystal of the present invention. In this case, it is preferable to configure the device by taking advantage of the characteristics of the ferroelectric crystal of the present invention that high characteristics can be obtained when an electric field is applied in the a-axis or b-axis direction.

  In the nonvolatile memory, the drive voltage can be lowered by using the ferroelectric of the present invention having a small coercive electric field Ec. Further, by using the ferroelectric of the present invention having a large remanent polarization Pr, a high-density memory device having a large storage capacity can be constructed.

  Examples of the piezoelectric device include an electronic lighter ignition device, an electronic speaker, an electronic buzzer, a piezoelectric transformer, an oscillator, a piezoelectric sensor, and a sonar. By using the ferroelectric of the present invention having a small coercive electric field Ec, the poling process is facilitated and the obtained piezoelectric characteristics are improved.

  Hereinafter, the present invention will be described in more detail with reference to examples.

  In Example 1, an example in which the ferroelectric crystal of the present invention was produced and its characteristics were measured will be described.

First, a single crystal of Bi ₄ Ti ₃ O ₁₂ was produced as the crystal (A) by the following method. First, 95 g of Bi ₂ O ₃ powder (manufactured by Kojundo Chemical Laboratory Co., Ltd., purity 99.9999% (metal molar ratio)) and 5 g of TiO ₂ powder (manufactured by Kojundo Chemical Laboratory Co., Ltd., 99.99% purity (metal molar ratio)) and mixed in a platinum crucible. Next, this crucible was heated at 1150 ° C. for 5 hours to melt. The melt of the material was cooled to 1000 ° C. at a cooling rate of −5 ° C./hour, and then gradually cooled to room temperature. The atmosphere during melting and cooling of the material was air. The obtained Bi ₄ Ti ₃ O ₁₂ single crystal was washed with nitric acid to remove the remaining metal Bi and Bi ₂ O ₃ , and then a rectangular parallelepiped test piece was cut out to measure the characteristics. The test piece had a length in the a-axis or b-axis direction of 0.1 to 0.2 mm, and a size perpendicular to the a-axis or b-axis direction was 0.1 mm × 1 mm.

  Next, the test piece was heat-treated in a 200 atmospheres pure oxygen gas atmosphere. Specifically, first, the test piece was heated from room temperature to 750 ° C. over 1.5 hours in pure oxygen gas of about 200 atm, and heated at a temperature of 750 ° C. for 5 hours. Next, the temperature was lowered over 12 hours until the temperature of the test piece reached 300 ° C. Next, the temperature of the test piece was lowered over 3 hours until it reached room temperature. The atmosphere during the temperature drop was also a pure oxygen gas atmosphere of about 200 atm. In addition, the temperature increase rate during temperature increase and the temperature decrease rate during temperature decrease were set to be substantially constant. Next, a gold electrode was formed by a sputtering method on a surface perpendicular to the a-axis or b-axis of the test piece after the heat treatment to obtain a sample 1 for measurement.

  Further, samples 2 and 3 were manufactured by performing heat treatment with the oxygen partial pressure during the heat treatment, that is, the pressure of the pure oxygen gas atmosphere being 1 atm or 10 atm. As a comparative example, a gold electrode was formed on a test piece that was not heat-treated, and Comparative Sample 1 was obtained.

  The hysteresis characteristics of the four types of measurement samples thus obtained were measured by the virtual ground method. The hysteresis was measured using a 1 Hz triangular wave AC voltage in 25 ° C. silicone oil. The measurement was performed by applying an electric field in the a-axis or b-axis direction. The measurement results are shown in FIGS. 1 and 2 and Table 1.

As shown in FIGS. 1 and 2, the single crystal of the present invention exhibits a substantially symmetrical hysteresis characteristic. As is apparent from the figure and table, the coercive electric field was decreased by heat treatment, and the remanent polarization was increased. This effect becomes more prominent as the oxygen partial pressure is higher. Ec is drastically reduced from 40 kV / cm to 20 kV / cm by performing heat treatment under the condition where the oxygen partial pressure is 200 atm, and the residual polarization Pr is 40 μC / cm. Increased from ² to 50 μC / cm ² .

In Example 2, the result of evaluating the influence of the purity of the material of the crystal (A) will be described. In this example, two types of Bi ₄ Ti ₃ O ₁₂ single crystals were produced using Bi ₂ O ₃ powders with a purity of 99.99% (metal molar ratio) and 99.9999% (metal molar ratio), Each was heat-treated in pure oxygen at 1 atm. Then, when an attempt is made to measure the hysteresis characteristics after heat treatment, purity _Bi 4 _Ti 3 _{O 12} single crystal was produced using a 99.99% _Bi 2 _{O 3} powder, hysteresis characteristics leakage current is too large, It was not possible to measure. The leakage currents of both are shown in FIG. As is clear from these results, it is important that the purity of the single crystal material is high. Even when Bi ₂ O ₃ powder having a purity of 99.999% (metal molar ratio) is used, the effects of the present invention can be obtained.

  Although the embodiments of the present invention have been described above by way of examples, the present invention is not limited to the above-described embodiments, and can be applied to other embodiments based on the technical idea of the present invention.

  The present invention can be applied to a ferroelectric and a device using the same, for example, a nonvolatile memory and a piezoelectric device.

Claims (4)

Forming a layered structure ferroelectric crystal (A) containing bismuth using a material containing at least one bismuth supply material selected from bismuth and a bismuth compound;
A heat treatment step of heat-treating the crystal (A) in an atmosphere having an oxygen partial pressure of 1 atm or more,
Wherein Ri der purity bismuth more than 99.999 mole% is included in at least one of bismuth feed materials,
A method for producing a ferroelectric crystal having a bismuth layer structure , wherein the crystal (A) is a Bi ₄ Ti ₃ O ₁₂ single crystal . 2. The method for producing a ferroelectric crystal having a bismuth layer structure according to claim 1, wherein the temperature of the heat treatment is 300 ° C. or higher and lower than the melting point of the crystal (A). 3. The bismuth layered structure according to claim 2, wherein the heat treatment step includes a step of heating the crystal (A) to a temperature higher than 300 ° C. and then cooling to 300 ° C. at a temperature lowering rate slower than −1 ° C./min. A method for producing a ferroelectric crystal. The method for producing a ferroelectric crystal having a bismuth layer structure according to any one of claims 1 to 3, wherein the oxygen partial pressure is 10 atm or more.

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