Method for producing piezoelectric single crystal ingot and piezoelectric single crystal ingot

Abstract

A method for producing a piezoelectric single crystal ingot shows small variation in the concentration of PbTiO.sub.3 in the growth direction of single crystal. A complete solid solution-type piezoelectric single crystal ingot is produced by using the Bridgman method, including: filling a starting material, wherein a relaxor having a compositional formula Pb(B.sub.1, B.sub.2)O.sub.3 is blended with lead titanate having a composition PbTiO.sub.3 to give a preset composition, into a crucible for growth; heating to the melting temperature to give a melted liquid layer; then moving the crucible for growth toward the low temperature side; and thus starting one-direction solidification from the lower part of the crucible to thereby produce a single crystal. During solidification, the feedstock containing the relaxor and lead titanate having a maximum grain size ≤3 mm is continuously supplied into the crucible.

Claims

1. A method for producing a solid solution piezoelectric single crystal ingot containing a relaxor having a composition formula of Pb(B.sub.1, B.sub.2)O.sub.3 and lead titanate having a composition of PbTiO.sub.3, using a Bridgman method, the production method comprising continuously feeding a raw material, which has a maximum particle size of 3 mm or less and contains the relaxor and the lead titanate, into a crucible for growth so that a concentration of the lead titanate is substantially constant in a growth direction of single crystal, and a variation range of the concentration is ±0.5 mol % or less over a length of 100 mm or more in the growth direction of single crystal, wherein B.sub.1 is at least one element selected from the group consisting of Zn, Mg, Ni, Sc, In and Yb, and B.sub.2 is at least one element selected from the group consisting of Nb and Ta.

2. A piezoelectric single crystal ingot produced by the production method according to claim 1, wherein a concentration of lead titanate is substantially constant, and a variation range of the concentration is ±0.5 mol % or less over a length of 100 mm or more in a growth direction of single crystal.

3. The piezoelectric single crystal ingot according to claim 2, wherein the piezoelectric single crystal ingot further contains 0.5 ppm by mass to 5% by mass in total of one or more selected from Cr, Mn, Fe, Li, Ca, Sr, Ba, Zr, and Sm when 1 mol of Pb is 100% by mass.

4. A piezoelectric device produced from the piezoelectric single crystal ingot according to claim 2.

5. A piezoelectric device produced from the piezoelectric single crystal ingot according to claim 3.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is an explanatory view (cross-sectional view) schematically illustrating a single crystal growth (manufacturing) apparatus suitable for carrying out the present invention.

(2) FIG. 2 is a graph illustrating an example of variation of concentration (mol %) of lead titanate along an ingot growth direction.

(3) FIG. 3 is a graph illustrating an example of variation of piezoelectric constant along the ingot growth direction.

(4) FIG. 4 is a graph illustrating an example of the variation of the concentration (mol %) of lead titanate along the ingot growth direction.

(5) FIG. 5 is a graph illustrating an example of the variation of the piezoelectric constant along the ingot growth direction.

(6) FIG. 6 is a graph illustrating an example of the variation of the concentration (mol %) of lead titanate along the ingot growth direction.

(7) FIG. 7 is a graph illustrating an example of the variation of the piezoelectric constant along the ingot growth direction.

DESCRIPTION OF EMBODIMENTS

(8) A piezoelectric single crystal ingot of the present invention is a solid solution piezoelectric single crystal ingot which contains a relaxor having a composition formula of Pb(B.sub.1, B.sub.2)O.sub.3 and lead titanate having a composition of PbTiO.sub.3 and has a perovskite structure. Examples thereof include a solid solution piezoelectric single crystal ingot (PMN-PT) containing a relaxor having a composition formula of Pb(Mg.sub.1/3Nb.sub.2/3)O.sub.3 and lead titanate having a composition of PbTiO.sub.3 and a solid solution piezoelectric single crystal ingot (PIN-PMN-PT) containing a relaxor having a composition formula of Pb(In.sub.1/2Nb.sub.1/2)O.sub.3 and Pb(Mg.sub.1/3Nb.sub.2/3)O.sub.3 and lead titanate having a composition of PbTiO.sub.3.

(9) The term “solid solution single crystal” as used herein refers to a single crystal in which two materials in the single crystal are completely dissolved in each other in a liquid state and completely make a solid solution together even in a solid state.

(10) The piezoelectric single crystal ingot of the present invention is produced using a vertical Bridgman method.

(11) An outline of the vertical Bridgman method is illustrated in FIG. 1. In the vertical Bridgman method, a raw material in a crucible for growth 20 is heated to a melting point or higher by using a heat source (heater) 50 to form a melt layer 40. Then the crucible for growth 20 is moved in the low temperature direction (the A direction in FIG. 1), and unidirectional solidification is started from a lower portion of the crucible to produce a single crystal 1. The crucible for growth 20 is installed on a lift mechanism 60 and is movable up and down in a furnace.

(12) When the piezoelectric single crystal 1 is produced using the vertical Bridgman method, first, an initial raw material is filled into the crucible for growth 20. Preferred examples of the initial raw material include a mixed powder obtained by blending and mixing a predetermined amount of a powder of a relaxor having a composition formula of Pb(B.sub.1, B.sub.2)O.sub.3 and a lead titanate powder having a composition of PbTiO.sub.3 so as to obtain a desired piezoelectric single crystal composition and a sintered polycrystalline body pellet of a relaxor-lead titanate solid solution. Furthermore, the initial raw material may be a pellet or the like obtained by sintering a mixture of a relaxor having a composition formula of Pb(B.sub.1, B.sub.2)O.sub.3 and lead titanate having a composition of PbTiO.sub.3 so as to obtain a desired piezoelectric single crystal composition.

(13) The crucible for growth 20 filled with the initial raw material is heated by the heater 50, and the internal initial raw material is melted to form the melt layer 40. Next, the crucible for growth 20 is moved in the arrow A direction, which is the low temperature direction, by the lift mechanism 60, whereby unidirectional solidification is started from the lower portion of the crucible to form the piezoelectric single crystal 1.

(14) The concentration of lead titanate in the single crystal 1 of the relaxor-lead titanate solid solution precipitated from the melt layer 40 varies according to a segregation coefficient k.sub.eff of lead titanate. k.sub.eff is a constant that defines a ratio of the concentration of lead titanate PbTiO.sub.3 in the solid solution to the concentration of lead titanate PbTiO.sub.3 in the melt layer 40. When k.sub.eff is different from 1, segregation proceeds in the single crystal 1 of the relaxor-lead titanate solid solution. For example, in the case of PMN-PT, the segregation coefficient k.sub.eff of lead titanate is less than 1, and as a solid solution single crystal grows, segregation of PbTiO.sub.3 occurs, the concentration of PbTiO.sub.3 in the melt layer 40 increases, and compositional variation in the melt layer 40 occurs. As a result, in the produced piezoelectric single crystal 1, segregation occurs where the concentration of PbTiO.sub.3 increases in a crystal growth direction. Also in the case of PIN-PMN-PT, the segregation coefficient k.sub.eff of lead titanate is less than 1.

(15) Thus, in the present invention, the compositional variation of the melt layer 40 in the crucible for growth 20 during single crystal growth is suppressed. For this purpose, a feedstock 30 is continuously dropped into a sub-crucible 70 in the crucible for growth 20 at a constant feed rate.

(16) The dropped feedstock 30 is heated and melted in the sub-crucible 70 and then added dropwise and fed to the melt layer 40. In the present invention, the feedstock 30 is a raw material having a maximum particle size of 3 mm or less. As a result, variation in the time of melting the feedstock 30 is reduced, and the melted feedstock can be added dropwise to the melt layer 40 at a constant feed rate. When the size of the feedstock 30 increases beyond 3 mm, the melting time in the sub-crucible 70 varies greatly, the amount of the melted feedstock fed to the melt layer 40 varies, and the feed rate is not constant. Thus, the compositional variation of the melt layer 40 increases, and the compositional variation in the growth direction in the obtained piezoelectric single crystal 1 increases. The size of the feedstock is more preferably in the range of 1 to 3 mm in consideration of ease of handling. Furthermore, the feedstock 30 is preferably a raw material having a composition where a PbTiO.sub.3 fraction is reduced as compared with the initial raw material.

(17) After the solidification is completed up to an upper portion of the crucible for growth 20, cooling is performed to room temperature to obtain a piezoelectric single crystal ingot 1. The obtained piezoelectric single crystal ingot 1 is an ingot in which the concentration of PbTiO.sub.3 is substantially constant and a variation range of the concentration is small. Specifically, it is possible to produce the piezoelectric single crystal ingot 1 which has a uniform composition and in which the PbTiO.sub.3 concentration is substantially constant and the variation range of the concentration is ±0.5 mol % or less over a length of 100 mm or more in the growth direction of single crystal.

(18) When a relative dielectric constant ε.sub.r or a mechanical quality factor Q.sub.m, which are properties of a piezoelectric single crystal device, must be high, the piezoelectric single crystal ingot described above preferably further contains 0.5 ppm by mass to 5% by mass in total of one or more selected from Cr, Mn, Fe, Li, Ca, Sr, Ba, Zr, and Sm when 1 mol of Pb is 100% by mass. When the content is less than 0.5 ppm by mass, the effect is not remarkable. On the other hand, when the content exceeds 5% by mass, the possibility of generating a polycrystal becomes large. Thus, when these elements are contained, the total content is preferably limited to the range of 0.5 ppm by mass to 5% by mass. Among these elements, the addition of Cr, Mn, and Fe effectively contributes to improvement of the mechanical quality factor and suppression of deterioration over time. Further, the addition of Li effectively contributes to suppression of formation of a polycrystalline region. Furthermore, the addition of Ca, Sr, Ba, Zr, and Sm effectively contributes to improvement of the relative dielectric constant sr.

(19) Hereinafter, the present invention will be further described with reference to Examples.

EXAMPLES

Example 1

(20) A PMN-PT piezoelectric single crystal ingot was produced using the vertical Bridgman method. A single crystal growth (manufacturing) apparatus used is illustrated in FIG. 1.

(21) As an initial raw material, 66% of PMN and 34% of PT were blended in terms of mass % relative to the total amount of the initial raw material, and 3,000 g of the mixed raw material was filled in the crucible for growth 20. The crucible for growth 20 was installed on the lift mechanism 60 in the furnace. Next, the crucible for growth 20 was heated with the heater 50 to 1,350° C. or higher, which is higher than the melting point, to melt the initial raw material 20 and thus to form the melt layer 40. Subsequently, the crucible for growth 20 was lowered in the A direction by the lift mechanism 60 to start solidification from a lower portion of the melt layer 40 in the crucible for growth 20.

(22) Next, the feedstock 30 was continuously dropped into the sub-crucible 70 in the crucible for growth 20 at a constant feed rate. As the dropped feedstock 30, a material obtained by blending and mixing 71% of PMN and 29% of PT in terms of mass % relative to the total amount of the feedstock was used. The feedstock 30 used had a maximum particle size of 3 mm or less. The particle size was measured by a sieving method. The “sieving method” as used herein was performed in accordance with the provisions of JIS Z 8815-1994.

(23) The dropped feedstock 30 was heated and melted in the sub-crucible 70 and then fed to the melt layer 40. The dropping was stopped once a certain amount of the feedstock 30 was dropped. When the length of the single crystal reached 220 mm, the descent of the crucible for growth 20 was stopped, and cooling was performed to room temperature. After cooling, the obtained piezoelectric single crystal ingot was taken out from the manufacturing apparatus and used as Invention Example 1.

(24) A piezoelectric single crystal ingot was produced as Comparative Example 1 under the same conditions as Invention Example 1 except that the feedstock 30 having a maximum particle size of 3 to 4 mm was charged into the sub-crucible 70.

(25) For the obtained piezoelectric single crystal ingot (diameter: about 80 mmϕ), a crystal orientation of the side face of the ingot was confirmed by an X-ray direction finder, and then the ingot was roughly cut by a cutting machine so that a wafer having a {001} orientation was obtained. Then, a precision cutting machine was used to cut out a piezoelectric device material of a desired size having a {100} orientation from the roughly cut wafer, and an X-ray fluorescence analyzer XRF was used to measure the concentration (mol %) of lead titanate PbTiO.sub.3 at each length position along the growth direction of single crystal. The obtained results are illustrated in FIG. 2.

(26) As seen from FIG. 2, in Invention Example 1, the concentration (mol %) of lead titanate PbTiO.sub.3 is substantially constant at each position in the growth direction of single crystal, and the variation range of the concentration (composition ratio) of lead titanate PbTiO.sub.3 is ±0.5 mol % or less over 100 mm or more in the growth direction of single crystal. On the other hand, Comparative Example 1 shows that the variation range of the concentration (composition ratio) of lead titanate PbTiO.sub.3 greatly varies by more than ±0.5 mol % in the growth direction of single crystal.

(27) A desired wafer was obtained by rough cutting from a piezoelectric single crystal ingot whose required crystallographic orientation was determined. Next, after the wafer was ground and polished to have a predetermined thickness, a piezoelectric single crystal device material was cut out from the wafer by a precision cutting machine. Electrodes were prepared on upper and lower surfaces of the obtained piezoelectric single crystal device material and subjected to polarization treatment to obtain a piezoelectric single crystal device. Next, each piezoelectric constant d.sub.33 of these piezoelectric single crystal devices was measured using a d.sub.33 meter. The obtained results are illustrated in FIG. 3.

(28) As seen from FIG. 3, in Invention Example 1, variation in the piezoelectric constant d.sub.33 is little at each position along the growth direction of single crystal, and it can be said that a uniform piezoelectric single crystal ingot is obtained from which a piezoelectric device having an excellent piezoelectric constant can be produced at high yield. On the other hand, in Comparative Example 1, the piezoelectric constant d.sub.33 varies greatly, and it cannot be said that a piezoelectric single crystal ingot is obtained from which a piezoelectric device having excellent piezoelectric properties can be produced at high yield.

Example 2

(29) A PIN-PMN-PT piezoelectric single crystal ingot was produced as in Example 1 using the vertical Bridgman method and used as Invention Example 2. The single crystal growth (manufacturing) apparatus illustrated in FIG. 1 was used as in Example 1.

(30) In Invention Example 2, as an initial raw material, 26% of PIN, 40% of PMN, and 34% of PT were blended in terms of mass % relative to the total amount of the initial raw material, and 3,000 g of the mixed raw material was used. As the feedstock 30, a material obtained by blending and mixing 26% of PIN, 45% of PMN, and 29% of PT in terms of mass % relative to the total amount of the feedstock was used. The feedstock 30 had a maximum particle size of 3 mm or less. The particle size was measured by a sieving method as in Example 1.

(31) In Invention Example 2, the piezoelectric single crystal ingot was produced using the above raw materials by the same method as in Invention Example 1. A piezoelectric single crystal device material and a piezoelectric single crystal device were produced from the ingot and evaluated as in Example 1. A piezoelectric single crystal ingot was produced as Comparative Example 2 under the same conditions as Invention Example 2 except that as the feedstock 30, a raw material having the same composition as that of Invention Example 2 and having a maximum particle size of 3 to 4 mm was used. A piezoelectric single crystal device material and a piezoelectric single crystal device were produced from the ingot and evaluated as described above. The obtained results are illustrated in FIGS. 4 and 5.

(32) As seen from FIG. 4, in Invention Example 2, as in Invention Example 1, the concentration (mol %) of lead titanate PbTiO.sub.3 is substantially constant at each position in the growth direction of single crystal, and the variation range of the concentration of lead titanate PbTiO.sub.3 is ±0.5 mol % or less over 100 mm or more in the growth direction of single crystal. On the other hand, Comparative Example 2 shows that the variation range of the concentration of lead titanate PbTiO.sub.3 greatly varies by more than ±0.5 mol % in the growth direction of single crystal.

(33) As seen from FIG. 5, in Invention Example 2, variation in the piezoelectric constant d.sub.33 is little at each position along the growth direction of single crystal, and it can be said that a uniform piezoelectric single crystal ingot is obtained from which a piezoelectric device having an excellent piezoelectric constant can be produced at high yield. On the other hand, in Comparative Example 2, the piezoelectric constant d.sub.33 varies greatly, and it cannot be said that a piezoelectric single crystal ingot is obtained from which a piezoelectric device having excellent piezoelectric properties can be produced at high yield.

Example 3

(34) An Mn-added PMN-PT piezoelectric single crystal ingot was produced as in Example 1 using the vertical Bridgman method and used as Invention Example 3. The single crystal growth (manufacturing) apparatus illustrated in FIG. 1 was used as in Example 1.

(35) In Invention Example 3, as an initial raw material, 9% of MnO, 57% of PMN, and 34% of PT were blended in terms of mass % relative to the total amount of the initial raw material, and 3,000 g of the mixed raw material was used. As the feedstock 30, a material obtained by blending and mixing 4% of MnO, 67% of PMN, and 29% of PT in terms of mass % relative to the total amount of the feedstock was used. The feedstock 30 had a maximum particle size of 3 mm or less. The particle size was measured by a sieving method as in Example 1.

(36) In Invention Example 3, the piezoelectric single crystal ingot was produced using the above raw materials by the same method as in Invention Example 1. A piezoelectric single crystal device material and a piezoelectric single crystal device were produced from the ingot and evaluated as in Example 1. A piezoelectric single crystal ingot was produced as Comparative Example 3 under the same conditions as Invention Example 3 except that as the feedstock 30, a raw material having the same composition as that of Invention Example 3 and having a maximum particle size of 3 to 4 mm was used. A piezoelectric single crystal device material and a piezoelectric single crystal device were produced from the ingot and evaluated as described above. The obtained results are illustrated in FIGS. 6 and 7.

(37) As seen from FIG. 6, in Invention Example 3, as in Invention Example 1, the concentration (mol %) of lead titanate PbTiO.sub.3 is substantially constant at each position in the growth direction of single crystal, and the variation range of the concentration of lead titanate PbTiO.sub.3 is ±0.5 mol % or less over 100 mm or more in the growth direction of single crystal. On the other hand, Comparative Example 3 shows that the variation range of the concentration of lead titanate PbTiO.sub.3 greatly varies by more than ±0.5 mol % in the growth direction of single crystal.

(38) As seen from FIG. 7, in Invention Example 3, variation in the piezoelectric constant d.sub.33 is little at each position along the growth direction of single crystal, and it can be said that a uniform piezoelectric single crystal ingot is obtained from which a piezoelectric device having an excellent piezoelectric constant can be produced at high yield. On the other hand, in Comparative Example 3, the piezoelectric constant d.sub.33 varies greatly, and it cannot be said that a piezoelectric single crystal ingot is obtained from which a piezoelectric device having excellent piezoelectric properties can be produced at high yield.

REFERENCE SIGNS LIST

(39) 1 Piezoelectric single crystal ingot (piezoelectric single crystal, single crystal) 10 Single crystal growth (manufacturing) apparatus 20 Crucible for growth 30 Feedstock 40 Melt layer 50 Heater (heat source) 60 Lift mechanism 70 Sub-crucible