Ferroelectric ceramic material

Abstract: Proposed are ferroelectric ceramic materials which are Perovskite solid solutions comprising a solid solution of formula Pb 1 - a M a (Mg 1/3 Nb 2/3 ) x Ti y Zr z O₃ wherein M is Ba or Sr, x + y + z is 1, a is from 0 to 0.10, x is from 0.05 to 0.70, y is from 0.25 to 0.50 and z is from 0.05 to 0.70, which solid solution contains either (i) at least one oxide selected from La₂O₃ and Nd₂O₃ as group A oxide and at least one oxide selected from ZnO, SnO₂ and Ta₂O₅ as group B oxide, or (ii) MnO₂, at least one said group A oxide and at least one oxide selected from ZnO and SnO₂ as group B oxide. These ferroelectric ceramic materials have a large piezoelectric strain constant d.

Description: The present invention relates to a ferroelectric ceramic material of Pb (Mg 1/3 Nb 2/3 ) O₃ - PbTiO₃ - PbZrO₃ series. More particularly, it relates to a ferroelectric ceramic material of Pb (Mg 1/3 Nb 2/3 ) O₧₃ - PbTiO₃ - PbZrO₃ series having a large piezoelectric strain constant d and an excellent mechanical quality factor Q m . Piezoelectric materials comprising ferroelectric ceramic materials have heretofore been used in piezoelectric filters, piezoelectric transducers, ultrasonic oscillators and piezoelectric buzzers. The most typical ferroelectric ceramic materials that have been used in such applications are solid solutions of PbTiO₃ - PbZrO₃ series. Solid solutions of Pb (Mg 1/3 Nb 2/3 ) O₃ - PbTiO₃ - PbZrO₃ series (Japanese Patent Publication No. 42-9716) and those further containing BaTiO₃, SrTiO₃ and/or CaTiO₃ are also known as having improved piezoelectric characteristics. On the other hand, use of piezoelectric ceramic materials as an actuator has also been recently studied. In this case, it is necessary to transform electric energy to mechanical energy by the displacement of the piezoelectric ceramic material itself by the so transformed mechanical energy. Accordingly, piezoelectric ceramic materials having a large piezoelectric strain constant d are desired. The piezoelectric strain constant d, is related to an electromechanical coupling factor k and a relative dielectric constant ε, of a piezoelectric ceramic material, as follows: d ∝ k √ε and therefore, in order that the material has a large piezoelectric strain constant d, it must have a large electromechanical coupling factor k and/or a large relative dielectric contant ε. Further, in applications of piezoelectric ceramic materials as a driving part of an actuator, for example, of an ultrasonic motor, where mechanical resonance of the material is utilized, it is desired that both the piezoelectric strain constant d and the mechanical quality factor Q m are large. When a piezoelectric ceramic material is used in an ultrasonic motor, if the mechanical quality factor Q m of the material is small, heat is generated due to high frequency driving involved, frequently leading to undesirable reduction in spontaneous polarization of the material and changes in the piezoelectric strain constant of the material with time. Accordingly, it is essential that the mechanical quality factor Q m of a piezoelectric ceramic material be large, when the material is to be used in ultrasonic motors. While the mechanical quality factor Q m of solid solutions of PbTiO₃ - PbZrO₃ series or of Pb(Mg 1/3 Nb 2/3 ) O₃ - PbTiO₃ - PbZrO₃ series can be improved by incorporation of MnO₂ thereinto, there has been a problem in that the piezoelectric strain constant d of the material is drastically decreased as the amount of the MnO₂ added increases. Accordingly, base solid solutions prior to the addition of MnO₂ thereto should preferably have a sufficiently large piezoelectric strain constant d. While various attempts have heretofore been made to add various oxides to solid solutions of Pb (Mg 1/3 Nb 2/3 ) O₃ - PbTiO₃ - PbZrO₃ series optionally containing BaTiO₃, SrTiO₃ and/or CaTiO₃ for a purpose of increasing the electromechanical coupling factor k and/or relative dielectric constant ε, of the solid solutions, thereby increasing the piezoelectric strain constant d of the materials, the attainable level of the piezoelectric strain constant d has not necessarily been satisfactory. For example, when a solid solution of Pb (Mg 1/3 Nb 2/3 ) O₃ - PbTiO₃ - PbZrO₃ series optionally containing BaTiO₃, SrTiO₃ and/or CaTiO₃ is incorporated with NiO, the piezoelectric strain constant d of the material is increased as the amount of NiO added is increased. However, when the amount of NiO added exceeds a certain limit, the piezoelectric strain constant d of the material is rather decreased. This is believed because whereas Ni ions preferentially enter B sites of the Perovskite crystals represented by ABO₃, if the amount of NiO admixed with the Perovskite crystals exceeds the certain limit, ions in A sites of the crystals becomes short and, in consequence, a part of the NiO admixed can no longer enter B sites of the Perovskite crystals. It has now been found that if Perovskite crystals are incorporated with metal ions which enter A sites of the crystals in addition to metal ions which enter B sites of the crystal, such as Ni ions, ferroelectric ceramic materials having a larger piezoelectric strain constant d are obtained when compared with a case wherein the Perovskite crystals are incorporated with metal ions which enter only B sites. It has also been found that if the so obtained ferroelectric ceramic solid solutions having a large piezoelectric strain constant d are further incorporated with MnO₂, ferroelectric ceramic materials having large piezoelectric strain constant d and mechanical quality factor Q m are obtained. The invention is based on the above-mentioned findings, and an object of the invention is to provide ferroelectric ceramic materials having a large piezoelectric strain constant d and an excellent mechanical quality factor Q m , and in consequence, which are excellent in piezoelectric characteristics and are suitable for use in an actuator. A first ferroelectric ceramic material according to the invention is a Perovskite solid solution comprising a solid solution represented by Pb 1-a M a (Mg 1/3 Nb 2/3 ) x Ti y Zr z O₃ wherein M is Ba or Sr, x + y + z is 1, a is from 0 to 0.10, x is from 0.05 to 0.70, y is from 0.25 to 0.50 and z is from 0.05 to 0.70, containing in said solution at least one oxide selected from the group A noted below and at least one oxide selected from the group B noted below. Group A : La₂O₃ and Nd₂O₃; Group B : ZnO, SnO₂ and Ta₂O₅. The lower limit of "a" for the first ceramic material according to the invention is 0.01 rather than 0 in so far as the contracting states Germany, France and the United Kingdom are concerned. This is because of EP-A-319 006 which forms part of the state of the art under Article 54(3) and (4) EPC and designates only Germany, France and the United Kingdom. EP-A-319 006 discloses a piezoelectric ceramic composition for actuators, which consists essentially of a composite of perovskite compounds represented by the formula Pb (1-x) La x [z{(Zn α Mg (1-α) ) 1/3 Nb 2/3 }+(1-z){Zr y Ti (1-y) }] (1-x) O₃ wherein 0.03 ≦ x ≦ 0.07, 0.50 ≦ y ≦ 0.65, 0 < z < 0.20 and 0 < α < 1. The first ferroelectric ceramic material according to the invention in which metal ions of oxides selected from the group A and metal ions of oxides selected from the group B are incorporated in A and B sites of the ABO₃ type Perovskite crystals, respectively, has a greatly improved piezoelectric strain constant d when compared with known ferroelectric ceramic materials. Accordingly, the first ferroelectric ceramic material according to the invention exhibits excellent characteristics when used in applications such as an actuator. The second ferroelectric ceramic material according to the invention is a Perovskite solid solution comprising a solid solution represented by Pb 1-a M a (Mg 1/3 Nb 2/3 ) x Ti y Zr z O₃ wherein M is Ba or Sr, x + y + z is 1, a is from 0 to 0.10, x is from 0.05 to 0.70, y is from 0.25 to 0.50 and z is from 0.05 to 0.70, containing in said solution at least one oxide selected from the group A noted below and at least one oxide selected from the group B noted below, said solid solution further containing in said solid solution MnO₂. Group A : La₂O₃ and Nd₂O₃; Group B : ZnO and SnO₂. The second ferroelectric ceramic material according to the invention in which metal ions of oxides selected from the group A and metal ions of oxides selected from the group B are incorporated in A and B sites of the Perovskite crystals, respectively, and in which MnO₂ is further incorporated, when compared with known ferroelectric ceramic materials, has an improved piezoelectric strain constant d, while retaining a comparable mechanical quality factor Q m . Accordingly, the second ferroelectric ceramic material according to the invention exhibits excellent characteristics when used in applications as a driving part of an actuator, such as an ultrasonic motor wherein mechanical resonance of the material is utilized. The ferroelectric ceramic materials according to the invention will now be described in detail. The first ferroelectric ceramic material according to the invention is a Perovskite solid solution comprising a solid solution represented by Pb 1-a M a (Mg 1/3 Nb 2/3 ) x Ti y Zr z O₃ wherein M is Ba or Sr, x + y + z is 1, a is from 0 to 0.10, preferably from 0.01 to 0.07, x is from 0.05 to 0.70, preferably from 0.10 to 0.60, y is from 0.25 to 0.50, preferably from 0.30 to 0.45, and z is from 0.05 to 0.70, preferably from 0.10 to 0.60, containing in said solution at least one oxide selected from the group A noted below and at least one oxide selected from the group B noted below. Group A : La₂O₃ and Nd₂O₃; Group B : ZnO, SnO₂ and Ta₂O₅. When the Perovskite crystals constituting the first ferroelectric ceramic material according to the invention are represented by ABO₃, a metal of at least one oxide selected from the group A is present in A sites of the crystals in the form of metal ions as noted below, and a metal of at least one oxide selected from the group B is present in B sites of the crystals in the form of metal ions as noted below. Group A : (La³⁺ or Nd 3+) Group B : (Zn²⁺, Sn⁴⁺ or Ta⁵⁺). In the solid solution represented by Pb 1 - a M a (Mg 1/3 Nb 2/3 ) x Ti y Zr z O₃ wherein M is Ba or Sr, metal ions of the group A are present preferably in an amount of from 0.5 to 5.0 atom equivalents based on 100 atom equivalents of the sum of Pb and M present in the solid solution. With such an amount of metal ions in the group A, a particularly improved piezoelectric strain constant d can be achieved. The metal ions of the group B are present in the Perovskite solid solution preferably in such an amount so that the relation : 0.5 ≦ N ≦ 5.0 is met, in which N is a defined by the equation : wherein n Ai and n Bj are valencies of metal ions of the oxides of groups A and B which are present in the solid solution, respectively, and x Ai and x Bj are atom equivalents of metal ions of the oxides of groups A and B which are present in the solid solution, respectively. With such an amount of metal ions of the group B, a particularly improved piezoelectric strain constant d can be achieved. The second ferroelectric ceramic material according to the invention is a Perovskite solid solution comprising a solid solution represented by Pb 1 - a M a (Mg 1/3 Nb 2/3 ) x Ti y Zr z O₃ wherein M is Ba or Sr, x + y + z is 1, a is from 0 to 0.10, preferably from 0.01 to 0.07, x is from 0.05 to 0.70, preferably from 0.10 to 0.60, y is from 0.25 to 0.50, preferably from 0.30 to 0.45, and z is from 0.05 to 0.70, preferably from 0.10 to 0.60, containing in said solution at least one oxide selected from the group A noted below and at least one oxide selected from the group B noted below, said solid solution further containing in said solution MnO₂. Group A : La₂O₃ and Nd₂O₃; Group B : ZnO and SnO₂. When the Perovskite crystals constituting the second ferroelectric ceramic material according to the invention are represented by ABO₃, a metal of at least one oxide selected from the group A is present in A sites of the crystals in the form of metal ions as noted below, and a metal of at least one oxide selected from the group B is present in B sites of the crystals in the form of metal ions as noted below. Group A : (La³⁺ or Nd 3+); and Group B : (Zn²⁺ or Sn⁴⁺). In the solid solution represented by Pb 1 - a M a (Mg 1/3 Nb 2/3 ) x Ti y Zr z O₃ wherein M is Ba or Sr, metal ions of the group A are present preferably in an amount of from 0.5 to 5.0 atom equivalents based on 100 atom equivalents of the sum of Pb and M present in the solid solution. With such an amount of metal ions of the group A, a particularly improved piezoelectric strain constant d can be achieved. The metal ions of the group B are present in the Perovskite solid solution in such an amount so that the relation : 0.5 ≦ N ≦ 5.0 is met, in which N is a defined by the equation : wherein n Ai and n Bj are valencies of metal ions of the oxides of groups A and B which are present in the solid solution, respectively, and x Ai and x Bj are atom equivalents of metals ions of the oxides of groups A and B which are present in the solid solution, respectively. With such an amount of metal ions of the group B, a particularly improved piezoelectric strain constant d can be achieved. The amount of MnO₂ contained in the second ferroelectric ceramic material according to the invention is preferably from 0.1 to 2.0 % by weight. With such an amount of MnO₂ contained, an improved mechanical quality factor Q m can be achieved, while retaining the piezoelectric strain constant d at a satisfactorily high level. The ferroelectric ceramic materials according to the invention can be prepared by admixing particulate metal compounds such as oxides and salts in such proportions which provide a desired composition when calcined, and calcining the admixture. Processes for preparing the starting particulate metal compounds are not particularly limited. They may be prepared by various known processes, including liquid phase processes such as precipitation, coprecipitation, alkoxide and sol-gel processes, and solid phase processes such as those based on decomposition of oxalates and blending of oxides. The admixture of particulate metal compounds in appropriate proportions may be pre-calcined at a temperature of from 800 to 1000 °C., pulverized in a ball mill, dried, pressed to a sheet under a pressure of from 50 to 150 MPa (500 to 1500 kg/cm²), and finally calcined at a temperature of from 1000 to 1300 °C., whereupon a desired ferroelectric ceramic material may be obtained. Examples While the invention will now be described by the following examples, it should be appreciated that the invention is in no way restricted to the examples. The radial electromechanical coupling factor K p , relative dielectric constant ε and piezoelectric strain constant d₃₁ and mechanical quality factor Qm of ferroelectric `ceramic materials were measured in accordance with Japan Electronic Material Manufactures Association Standard (EMAS). Examples 1 to 5, and Comparative Examples 1 PbO, ZrO₂, TiO₂, MgCO₃, Nb₂O₅, SrCO₃, La₂O₃, ZnO and SnO₂ were weighed in proportions which provide each composition as indicated in Table 1, pulverized and admixed in a ball mill. The pulverized admixture was pre-calcined at a temperature of from 800 to 1000 °C. for a period of from 1 to 2 hours, pulverized in a ball mill and dried. The mixture was then pressed to a disc having a diameter of 25 mm under a pressure of about 100 MPa (1000 kg/cm²) and finally calcined at a temperature of from 1050 to 1250 °C. for a period of from 1 to 2 hours. The calcined disc so prepared was polished to a thickness of 0.5 mm, coated with silver paste on both surfaces and baked. It was then polarized by application of a DC electric field of from 20 to 40 KV/cm in a silicone oil and thereafter aged for 12 hours. The specimen so prepared was tested for various electric properties. The results are shown in Table 1. In Table 1, a, x, y and z represent coefficients appearing in the formula: Pb 1 - a Sr a (Mg 1/3 Nb 2/3 ) x Ti y Zr z O₃ [x + y + z = 1], p, q and r represent atom equivalents of respective metal ions indicated in the same table based on 100 atom equivalents of the sum of Pb and Sr, and N is a value defined by the equation : wherein n Ai and n Bj are valencies of metal ions of the groups A and B, respectively, and x Ai and x Bj are atom equivalents of metal ions of the groups A and B, respectively. It is revealed from Table 1 that when compared with the ferroelectric ceramic material [I] of Comparative Example 1 having a composition of the formula : Pb 0.95 Sr 0.05 (Mg 1/3 Nb 2/3 ) 0.375 Ti 0.375 Zr 0.250 O₃, the ferroelectric ceramic materials of Examples 1 to 5 according to the invention, which comprise the ferroelectric ceramic material [I] having incorporated with La³⁺ in the A sites and with Zn²⁺, or Zn²⁺ and Sn⁴⁺ in the B sites, do not have a larger piezoelectric strain constant d₃₁. Examples 6 to 9 PbO, ZrO₂, TiO₂, MgCO₃, Nb₂O₅, SrCO₃, at least one oxide selected from the group A consisting of La₂O₃ and Nd₂O₃, and at least one oxide selected from the group B consisting of ZnO, SnO₂ and Ta₂O₅, were weighed in proportions which provide each composition as indicated in Table 2, pulverized and admixed in a ball mill. The pulverized admixture was processed as in Example 1 to prepare a ferroelectric ceramic material, which was tested for electric properties. The results are shown in Table 2. Table 2 also reveals that joint inclusion of metal ions of the groups A and B into the ferroelectric ceramic material [I] improves the piezoelectric strain constant d₃₁. Examples 10 to 12, and Comparative Examples 2 to 4 Each of the ferroelectric ceramic materials of the formula : Pb 1 - a Sr a (Mg 1/3 Nb 2/3 ) x Ti y Zr z O₃ [x + y + Z = 1], wherein the coefficients a, x, y and z are different from those of the ferroelectric ceramic material used in Examples 1 to 9, as indicated in Table 3, with (Examples 10 to 12) or without (Comparative Examples 2 to 4) La³⁺, Zn²⁺ and Sn⁴⁺ in amounts indicated in Table 3 incorporated therein, was prepared and tested for electric properties in the manner as described in Example 1. The results are shown in Table 3. Example 13 and Comparative Example 5 Ferroelectric ceramic materials were prepared and tested in the same manner as described in Example 1 and Comparative Example 1, except that the SrCO₃ was replaced with the equimolar amount of BaCO₃. The results are shown in Table 3. Examples 14 to 16, and Comparative Example 6 PbO, ZrO₂, TiO₂, MgCO₃, Nb₂O₅, SrCO₃, La₂O₃, ZnO, SnO₂ and MnO₂ were weighed in proportions which provide each composition as indicated in Table 4, pulverized and admixed in a ball mill. The pulverized admixture was processed as in Example 1 to prepare of a ferroelectric ceramic material, which was tested for various electric properties. The results are shown in Table 4. Table 4 reveals that in the ferroelectric ceramic material [I] having a composition of the formula : Pb 0.95 Sr 0.05 (Mg 1/3 Nb 2/3 ) 0.375 Ti 0.375 Zr 0.250 O₃, having incorporated with La³⁺ in the A sites and with Zn²⁺ and Sn⁴⁺ in the B sites, as the amount of MnO₂ incorporated is increased, the mechanical quality factor Q m of the material is increased while the piezoelectric strain constant d₃₁ of the material is decreased. It has been confirmed, however, that when compared with the ferroelectric ceramic material [I] having incorporated with MnO₂, according to Comparative Example 6, the ferroelectric ceramic materials according to the invention whose mechanical quality factor Q m have been increased to a comparable level of that of the material of Comparative Example 6 by addition of MnO₂, still have a larger piezoelectric strain constant d₃₁. Example 17 Example 15 was repeated except that the La₂O₃ was replaced with the equimolar amount of Nd₂O₃ to prepare a ferroelectric ceramic material, which was tested for electric properties. The results are shown in Table 5. Examples 18 to 20, and Comparative Examples 7 to 9 The ferroelectric ceramic materials of the formula : Pb 1 - a Sr a (Mg 1/3 Nb 2/3 ) x Ti y Zr z O₃ [x + y + Z = 1], wherein the coefficients a, x, y and z are different from those of the ferroelectric ceramic material used in Examples 14 to 17, as indicated in Table 5, with (Examples 18 to 20) or without (Comparative Examples 7 to 9) La³⁺, Zn²⁺ and Sn⁴⁺ in amounts indicated in Table 5 incorporated therein and containing 0.5 % by weight of MnO₂ were prepared and tested for electric properties in the manner as described in Example 1. The results are shown in Table 5. Example 21 and Comparative Example 10 Ferroelectric ceramic materials were prepared and tested in the same manner as described in Example 15 and Comparative Example 6, except that the SrCO₃ was replaced with the equimolar amount of BaCO₃. The results are shown in Table 5.

Claims: Claims for the following Contracting States : IT, NL, BE, CH, LI, AT, SE, LU, ES, GR A ferroelectric ceramic material which is a Perovskite solid solution which comprises a solid solution of formula Pb 1-a M a (Mg 1/3 Nb 2/3 ) x Ti y Zr z O₃ wherein M is Ba or Sr, x + y + z is 1, a is from 0 to 0.10, x is from 0.05 to 0.70, y is from 0.25 to 0.50 and z is from 0.05 to 0.70, which solid solution contains at least one oxide selected from La₂O₃ and Nd₂O₃ as group A oxide and at least one oxide selected from ZnO, SnO₂ and Ta₂O₅ as group B oxide. A ceramic material according to claim 1 wherein metal of said group B oxide is present in the form of Zn²⁺, Sn⁴⁺ or Ta⁵⁺ ions. A ferroelectric ceramic material which is a Perovskite solid solution which comprises a solid solution of formula Pb 1-a M a (Mg 1/3 Nb 2/3 ) x Ti y Zr z O₃ wherein M is Ba or Sr, x + y + z is 1, a is from 0 to 0.10, x is from 0.05 to 0.70, y is from 0.25 to 0.50 and z is from 0.05 to 0.70, which solid solution contains MnO₂, at least one oxide selected from La₂O₃ and Nd₂O₃ as group A oxide and at least one oxide selected from ZnO and SnO₂ as group B oxide. A ceramic material according to claim 3 wherein metal of said group B oxide is present in the form of Zn²⁺ or Sn⁴⁺ ions. A ceramic material according to claim 3 or 4 wherein the amount of MnO₂ present in the solid solution is from 0.1 to 2.0% by weight. A ceramic material according to any one of the preceding claims wherein metal of said group A oxide is present in the form of La³⁺ or Nd³⁺ ions. A ceramic material according to claim 6 wherein the metal ions of said group A oxide are present in an amount of from 0.5 to 5.0 atom equivalents per 100 atom equivalents of the sum of Pb and M present in the Perovskite solid solution. A ceramic material according to any one of the preceding claims wherein the metal ions of the group B oxide are present in such amount that the relation: 0.5≦N≦5.0 is met, in which N is defined by the equation: wherein n Ai and n Bj are the valencies of metal ions of group A and group B oxides, respectively, and x Ai and x Bj are the atom equivalents of metal ions of group A and group B oxides, respectively. Use of a ferroelectric ceramic material as claimed in any one of the preceding claims as an actuator. Claims for the following Contracting States : DE, FR, GB A ferroelectric ceramic material which is a Perovskite solid solution which comprises a solid solution of formula Pb 1-a M a (Mg 1/3 Nb 2/3 ) x Ti y Zr z O₃ wherein M is Ba or Sr, x + y + z is 1, a is from 0.01 to 0.10, x is from 0.05 to 0.70, y is from 0.25 to 0.50 and z is from 0.05 to 0.70, which solid solution contains at least one oxide selected from La₂O₃ and Nd₂O₃ as group A oxide and at least one oxide selected from ZnO, SnO₂ and Ta₂O₅ as group B oxide. A ceramic material according to claim 1 wherein metal of said group B oxide is present in the form of Zn²⁺, Sn⁴⁺ or Ta⁵⁺ ions. A ferroelectric ceramic material which is a Perovskite solid solution which comprises a solid solution of formula Pb 1-a M a (Mg 1/3 Nb 2/3 ) x Ti y Zr z O₃ wherein M is Ba or Sr, x + y + z is 1, a is from 0 to 0.10, x is from 0.05 to 0.70, y is from 0.25 to 0.50 and z is from 0.05 to 0.70, which solid solution contains MnO₂, at least one oxide selected from La₂O₃ and Nd₂O₃ as group A oxide and at least one oxide selected from ZnO and SnO₂ as group B oxide. A ceramic material according to claim 3 wherein metal of said group B oxide is present in the form of Zn²⁺ or Sn⁴⁺ ions. A ceramic material according to claim 3 or 4 wherein the amount of MnO₂ present in the solid solution is from 0.1 to 2.0% by weight. A ceramic material according to any one of the preceding claims wherein metal of said group A oxide is present in the form of La³⁺ or Nd³⁺ ions. A ceramic material according to claim 6 wherein the metal ions of said group A oxide are present in on amount of from 0.5 to 5.0 atom equivalents per 100 atom equivalents of the sum of Pb and M present in the Perovskite solid solution. A ceramic material according to any one of the preceding claims wherein the metal ions of the group B oxide are present in such amount that the relation: 0.5≦N≦5.0 is met, in which N is defined by the equation: wherein n Ai and n Bj are the valencies of metal ions of group A and group B oxides, respectively, and x Ai and x Bj are the atom equivalents of metal ions of group A and group B oxides, respectively. Use of a ferroelectric ceramic material as claimed in any one of the preceding claims as an actuator.

Inventors: HIRAI NOBUKI MITSUI PETROCHEMI, MASE HIROSHI MITSUI PETROCHEMI, USHIDA YOSHIHISA MITSUI PETROC, HIRAI, NOBUKI MITSUI PETROCHEMICAL IND., LTD., MASE, HIROSHI MITSUI PETROCHEMICAL IND., LTD., USHIDA, YOSHIHISA MITSUI PETROCHEMICAL IND., LTD.
Applicants: MITSUI PETROCHEMICAL IND, MITSUI PETROCHEMICAL INDUSTRIES, LTD.
Application year: 1989
Main classification: C04B35
IPC codes: H01L41, C04B35