WO1989010813A1 - Ceramic electrode material and electrical devices formed therewith - Google Patents

Ceramic electrode material and electrical devices formed therewith Download PDF

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Publication number
WO1989010813A1
WO1989010813A1 PCT/US1989/002066 US8902066W WO8910813A1 WO 1989010813 A1 WO1989010813 A1 WO 1989010813A1 US 8902066 W US8902066 W US 8902066W WO 8910813 A1 WO8910813 A1 WO 8910813A1
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Prior art keywords
ceramic
electrical device
ceramic composition
compositions
electrical
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PCT/US1989/002066
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French (fr)
Inventor
Amar S. Bhalla
Leslie E. Cross
Thomas R. Shrout
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Research Corporation Technologies, Inc.
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Publication of WO1989010813A1 publication Critical patent/WO1989010813A1/en

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES OR LIGHT-SENSITIVE DEVICES, OF THE ELECTROLYTIC TYPE
    • H01G4/00—Fixed capacitors; Processes of their manufacture
    • H01G4/002—Details
    • H01G4/005—Electrodes
    • H01G4/008—Selection of materials
    • H01G4/0085—Fried electrodes
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B06—GENERATING OR TRANSMITTING MECHANICAL VIBRATIONS IN GENERAL
    • B06B—METHODS OR APPARATUS FOR GENERATING OR TRANSMITTING MECHANICAL VIBRATIONS OF INFRASONIC, SONIC, OR ULTRASONIC FREQUENCY, e.g. FOR PERFORMING MECHANICAL WORK IN GENERAL
    • B06B1/00—Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency
    • B06B1/02—Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency making use of electrical energy
    • B06B1/06—Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency making use of electrical energy operating with piezoelectric effect or with electrostriction
    • B06B1/0644—Methods or apparatus for generating mechanical vibrations of infrasonic, sonic, or ultrasonic frequency making use of electrical energy operating with piezoelectric effect or with electrostriction using a single piezoelectric element
    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
    • C04B35/01—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on oxide ceramics
    • C04B35/45—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on oxide ceramics based on copper oxide or solid solutions thereof with other oxides
    • C04B35/4504—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on oxide ceramics based on copper oxide or solid solutions thereof with other oxides containing rare earth oxides
    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
    • C04B35/01—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on oxide ceramics
    • C04B35/45—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on oxide ceramics based on copper oxide or solid solutions thereof with other oxides
    • C04B35/4512—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on oxide ceramics based on copper oxide or solid solutions thereof with other oxides containing thallium oxide
    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
    • C04B35/01—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on oxide ceramics
    • C04B35/45—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on oxide ceramics based on copper oxide or solid solutions thereof with other oxides
    • C04B35/4521—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on oxide ceramics based on copper oxide or solid solutions thereof with other oxides containing bismuth oxide
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N30/00—Piezoelectric or electrostrictive devices
    • H10N30/80—Constructional details
    • H10N30/87—Electrodes or interconnections, e.g. leads or terminals
    • H10N30/871—Single-layered electrodes of multilayer piezoelectric or electrostrictive devices, e.g. internal electrodes
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N30/00—Piezoelectric or electrostrictive devices
    • H10N30/80—Constructional details
    • H10N30/87—Electrodes or interconnections, e.g. leads or terminals
    • H10N30/877—Conductive materials
    • H10N30/878—Conductive materials the principal material being non-metallic, e.g. oxide or carbon based

Definitions

  • This invention relates to ceramic material useful for forming electrodes in electrical devices and more particularly, to ceramic compositions being electrical conductors at ambient temperature.
  • Ceramic materials have been used for many years as insulators or dielectrics, having capactive, piezoelectric, ferroelectric and electrostrictive properties.
  • the ceramic insulators are used in the fabrication of electrical devices, such as, capacitors, transducers and actuators.
  • the devices are generally manufactured by depositing metal electrodes in the form of a paste or ink on opposing sides of the ceramic insulating material and firing the composite to complete the device.
  • the use of ceramics has grown with the introduction of multilayer ceramic devices.
  • multilayer type alternate layers of ceramic insulating and electrode materials are formed by a sequential or a lamination technique.
  • the metal electrodes which are generally selected from a group of precious metals based systems, can represent as much as 75% of the cost of the multilayer devices.
  • the devices are often fired at temperatures of 900°C or above which results in delamination problems between the precious metal electrodes and the insulating material. If non-precious metals .are utilized, a
  • Ceramic materials have been used in the prior art as electrodes in liquid crystal displays to apply an electric field to the liquid crystals.
  • Other uses for ceramics are as ionic conductors in high temperature fuel cells and in electrochlorination systems.
  • U.S. Patent 4,316,785 is directed to a Josephson Junction utilizing a superconducting oxide material of BaPb 1-x Bi x O 3 as thin film electrode layers sputtered onto an oxide barrier layer.
  • the barrier layer acts like a semiconductor to allow charge carriers to tunnel through the layer.
  • the electrode and barrier layers both have perovskite crystalline structures, favorably affecting the stability of the device.
  • the Josephson junction tunneling effect only occurs in materials that are
  • the oxide material disclosed is useful as an electrode only at very low temperatures, such as 13°K.
  • a ceramic material being an electrical conductor at room temperature for connection to a ceramic insulator for forming a single or multilayer electrical device.
  • the present invention is directed to ceramic compositions that are electrical conductors at ambient temperatures for forming ceramic electrodes for connection to a ceramic insulator or dielectric.
  • the ceramic electrodes are electrically coupled to the ceramic insulator to produce electrical devices such as single and multilayer capacitors, piezoelectric transducers and electrostriction devices.
  • the ceramic compositions useful for forming the room temperature electrodes of the invention are ceramics exhibiting superconductivity at low temperatures.
  • the ceramic superconductor materials especially useful as electrodes for electrical devices are those having a critical temperature above 50°K. Although the ceramic electrode materials are superconductors, the material need not be superconducting for the electrical devices to operate properly in their intended purpose.
  • the ceramic electrodes of the invention have sufficient conductivity above the superconducting critical temperature to function effectively as electrodes at room temperature.
  • the superconductor ceramic materials that are useful as electrodes may be selected from the YBaCuO system and the LaBaCuO system.
  • the electrodes may be made from materials of the BiSrCaCuO system and the TIBaCaCuO system. Ceramic compositions having higher critical temperatures are being discovered almost daily and any such composition may be utilized in the present invention.
  • An example of the particular phases of the high Tc ceramic superconductor materials that are especially useful as electrodes are ceramics having the general formula ZBa 2 Cu 3 O 9-y , where Z is Y, La or any of the other elements in the lanthanide series and where y is about 2.
  • the elements in the lanthanide series are known as element numbers 58 through 71 of the periodic table of the elements.
  • the electrical devices of the invention comprise at least one layer of an electrical insulating ceramic composition and at least one layer of an electrical conducting ceramic composition electrically coupled to the ceramic insulating layer.
  • the conducting ceramic composition is an electrical conductor at room temperature.
  • an electrical conductor is a body so constructed that it may be used as a carrier of electric current. In ordinary usage, a conductor is a material of relatively high conductivity.
  • An insulator is a material when placed between conductors at different potentials, permits only a small or negligible current to flow through it.
  • the term dielectric is synonomous with electrical insulator.
  • an insulator is a material of relatively high resistivity. Resistivity is inversely proportional to conductivity.
  • the ceramic electrodes of the invention have a resistivity in the range of 100-500 ⁇ ohms-cm at room temperature which is comparable to gold electrodes at ambient temperatures.
  • multilayer electrical devices such as capacitors, transducers, resonators and actuators are manufactured with a dielectric made from ceramic materials having a perovskite crystalline structure.
  • many multilayer capacitors have dielectrics based on barium, such as BaTiO 3 , which have a perovskite structure.
  • the ceramic electrodes of the present invention have a similar perovskite structure and are therefore more compatible with the ceramic dielectrics than the presently used metal electrode formulations. In addition, the ceramic electrodes are substantially less costly than the metal electrodes.
  • the ceramic electrodes of the invention and insulating materials forming the electrical devices of the present invention have similar thermal expansion, latticeconstant and other related chemical properties eliminating existing processing problems of thermal expansion mismatch and delamination in the manufacture of multilayer electrical devices. It is well known that device performance is highly dependent on the integrity of the insulator, since any voids or defects in the insulator will significantly reduce efficiency and reliability of the device. Thus, the electrical devices of the present invention have an overall increase in device reliability from. the use of the ceramic electrode materials disclosed herein.
  • the oxidation firing conditions for the ceramic electrode materials are similar to the ceramic dielectrics allowing for coprocessing of the electrodes and the insulating materials which will improve device reliability by enhancing defect chemistry control and also having the potential to reduce manufacturing costs.
  • the compatibility between the layers of the devices is optimized by combining the ceramic superconductor compositions containing barium with the barium based dielectrics.
  • the ceramic superconductor compositions containing bismuth are more compatible with bismuth based dielectrics.
  • the inventors have discovered that the barium containing ceramic electrode compositions may be modified by replacing a small percentage of the barium with one of a group of other elements to provide new superconducting compositions.
  • the novel ceramic compositions have the formula ZBa 2-x A x Cu 3 O 9-y where Z is Y, La or any of the other lanthanide series elements; A is Na, K, Rb, Cs, or Pb; x is in the range of 0.01 to 0.2; and y is about 2.
  • the new compositions are especially useful as electrodes in the single or multilayer electrical devices described above which have insulating materials containing Pb, Na, K, Rb or Cs.
  • the element substituted for Ba in the electrode is selected to match the element contained in the insulator to provide further compatibility between the layers of the device.
  • Figure 1 is a side sectional view of a ceramic electrical device of the present invention.
  • Figure 2 is a side sectional view of a multilayer electrical device of the present invention.
  • Figure 3 is a graph of dielectric constant versus temperature for one embodiment of a multilayer capacitor of the present invention.
  • Figures 4a and 4b are graphs of resistance versus temperature for one superconducting composition of the present invention before (4a) and after (4b) exposure to an oxidizing environment.
  • the ceramic electrode materials of the present invention are electrical conductors at room temperature and are useful as electrodes for single and multilayer electrical devices.
  • the ceramic electrodes are coupled to a ceramic insulating material to form capacitors, transducers, actuators and the like.
  • known ceramic compositions that are room temperature suitable as ceramic electrodes are high critical temperature, superconductor ceramic compositions.
  • the ceramic superconductor materials having critical temperatures above 50°K have been found to have sufficient conductivity at ambient temperatures and a similar crystalline structure to the ceramic dielectric to produce electrical devices having excellent performance characteristics.
  • compositions useful as electrode materials are: the ZBaCuO system where Z is Y,
  • any high Tc ceramic superconducting composition may be used as the electrode material coupled to an insulating ceramic material to form an electrical device of the invention.
  • Figure 1 shows a single layer device 10 having a ceramic insulating body or dielectric 12 sandwiched between a pair of ceramic superconducting electrodes 14 and 16 electrically coupled to the insulating body 12.
  • Figure 2 shows a multilayer electrical device 20 in which alternating layers of dielectric 22 and opposing electrodes 24 and 26 form the device. A pair of conductive terminations 28 and 30 provide the electrical connection between the electrodes 24 and 26 respectively.
  • the devices 10 and 20 of Figures 1 and 2 may form a capacitor, or any other piezoelectric or electrostrictive device, such as, a resonator or an actuator.
  • devices 10 and 20 are ceramic capacitors
  • the opposing electrodes, 14 and 16 of device 10 and the opposing electrodes 24 and 26 of device 20 carry current to and from the subject device for storage, release or regulation of electrical energy.
  • the electrical energy is stored in the dielectric material and the relative ability of particular materials to store-.electrical energy is defined by its relative permittivity or dielectric constant.
  • the capacitance is a function of the dielectric constant and capacitor performance is highly dependent upon the integrity of the dielectric.
  • dielectric materials especially for capacitors and piezoelectric devices
  • these ceramic dielectrics have a perovskite crystalline structure and require high sintering temperatures much higher than 950°C.
  • the electrodes are formulated into a paste or ink and silk-screened or painted on the dielectric layers.
  • capacitors having metal electrodes would suffer from delamination of the electrode layers due, in part, to the different sintering shrinkages between the metal and ceramic materials.
  • the interaction between the metal electrodes and ceramic dielectrics at the metal-ceramic interface would adversely alter the dielectric properties of the insulator.
  • the number of capacitor material layers has to be increased and hence the number of electrode layers must also be increased.
  • the main cost of most of these devices comes from the precious metal electrode materials and can, in some devices, carry as much as 75% of the cost of the final component.
  • the ceramic compositions of the present invention have the same perovskite crystalline structure as the insulating material.
  • the electrode and insulating material axe therefore compatible in chemistry, thermal expansion, lattice constant and other properties which facilitates processing flexibility and optimization and improves device performance.
  • the use of the ceramic compositions of the present invention significantly reduces the material cost and the cost of production of the devices.
  • the electrode materials are. ceramic
  • the oxidation firing conditions usually required for the multilayer processing are favorable with the ceramic superconductor electrodes.
  • device fabrication is facilitated as coprocessing of the electrodes and insulating materials can be easily performed.
  • the interfaces between the active insulating material and the superconductor electrodes are much more compatible.
  • the delamination problem in multilayer capacitors will be substantially eliminated.
  • a slight modification of the superconductor composition during processing does not change the electrical properties of the interface of the composite devices.
  • Dielectric defect chemistry is better controlled than in existing systems.
  • the reduction in voids in the dielectric material after processing results in an increase in the overall. device reliability.
  • the superconductor ceramic materials can be prepared as electrodes having a resistivity of approximately 100 to 500 ⁇ ohms-cm at ambient temperatures. As the resistivity is inversely proportional to conductivity, the very low range of resistivity translates into a high range of .conductivity.
  • ceramic superconductor compositions that can be used as electrode materials in forming electrical devices are ZBa 2 Cu 3 O 9-y , where Z is Y, La or any of the lanthanide series elements and where y is about 2.
  • Another ceramic superconductor composition suitable for electro d e materials is Bi 2 Sr 3-x Ca x Cu 2 O y , where x is from 0.0 to 2.0 and y is from about 5.0 to about 8.0.
  • the preparation of one ceramic electrode composition of the invention begins with a mixture of Y 2 O 3 , BaO 2 and CuO.
  • the starting materials are mixed in the molar ratio as follows: 1 ⁇ 2Y 2 O 3 + 2BaO 2 + 3CuO.
  • the mixture is ball milled and calcined at temperatures of about 920°C to 940°C for about 6 to 12 hours.
  • the mixture is then pressed into pellet form and heated at a temperature of about 940°C to 960°C for about 6 to 12 hours. After oxidation, the resulting composition has the formula YBa 2 Cu 3 O 7 .
  • Figure 3 shows the measurements taken of the dielectric constant versus temperature, at 1KHz, of a multilayer capacitor built having electrodes of the formula YBa 2 Cu 3 O 7 coupled to a BaTiO 3 dielectric using conventional capacitor processing techniques.
  • the dielectric constant is over 2000, which is similar to capacitors having gold electrodes.
  • the dielectric loss is shown as about 0.01 at room temperature. Thus, dielectric integrity is maintained after processing.
  • ferroelectric materials are lead magnesium niobate, Pb (Mg 1/3 Nb 2/3 )O 3 for electrostrictive devices and capacitors; lead iron niobate tungstate (Pb (FeNb)O 3 -Pb (FeW)O 3 and lead zinc niobate Pb(ZnNb)O 3 for piezoelectric, electrictive and capacitor applications and other materials such as, PbTiO 3 and
  • the ZBa 2 Cu 3 O 9-y composition may be modified by replacing a small percentage of the Ba with one of the elements Pb, Na, K, Rb or Cs and that the resulting modified compositions have excellent room temperature conductivity properties while remaining superconductors at low temperatures.
  • the inventors have further determined that by selecting the element for A to match the element upon which the insulator is based provides additional compatibility between the electrode and insulator layers, further enhancing device performance and reliability.
  • the novel superconductor composition has the general formula ZBa 2-x A x Cu 3 O 9-y , where Z is Y, La or any lanthanide series elements; A is Pb, Na, K, Rb or Cs; -x is in the range 0.01 to 0.2; and y is about 2.
  • a sample was prepared utilizing 10% Pb replacement for the Ba in the composition of. the invention.
  • x is equal to 0.2.
  • the starting materials were mixed in the molar ratios 0.5Y 2 O 3 + 1.8 BaO 2 + 0.2 PbO + 3 CuO.
  • the materials were ball milled and then calcined at 920°C for 10 hours.
  • the mixture was then pressed into pellet form and heated slowly at 920°C for 6 hours.
  • the pellets were then kept at 920°C for 6 hours.
  • the pellets were then cooled slowly to 600°C at approximately 10°C per minute and kept at 600°C for 2 hours.
  • the pellets were then cooled to room temperature at the rate of approximately 4°C per minute.
  • the pellets had a composition of the formula YBa 1.8 Pb. 2 Cu 3 O 9-y , where y was approximately 2.
  • the pellets were then oxidized in an oxygen rich atmosphere at 600°C for 10 hours and cooled to room temperature to simulate the capacitor firing conditions. Resistance versus temperature measurements were taken and are shown in Figures 4a and 4b.
  • Figure 4a shows the measurements before oxidation
  • Figure 4b shows the measurements after oxidation.
  • the resistance was measured as about 8 ⁇ ohms.
  • the resistance at room temperature was about 7 ⁇ ohms.
  • the resistivity was calculated to be about 533 ⁇ ohms-cm and 466 ⁇ ohms per cm, respectively. In other samples, the resistivity can be lowered by a factor of 4-5.
  • the range of resistivity of the electrodes of the invention is therefore from 100-500 ⁇ ohms-cm.
  • the ceramic electrodes have excellent room temperature conductivity both before and after oxidation.
  • the inventors have determined that by adding a small amount, approximately 1%, of either K 2 CO 3 or Rb 2 CO 3 or CsCo 3 with PbO during the initial preparation of the sample, helps densify the material without loosing any of the electrical conducting properties.
  • the density is always greater than 90% of the theoretical density of the pure YBa 2 Cu 3 O 9-y composition.
  • the theoretical density of the pure material is 6.38 grams per cc.
  • the ceramic electrode compositions of the present invention can be easily formulated into an ink or paste and applied to the insulator material during processing of the multilayer capacitors utilizing silk screening or painting operations similar to those currently used.
  • a significant reduction in the number of steps necessary for producing multilayer capacitors is made possible by the present invention by the co-firing of the electrodes and the ceramic dielectric. Many of the dielectrics must be fired above 900°C.
  • the ceramic material of the present invention reduces the number of preparation steps since the ceramic electrode can be fired at high temperatures with the dielectrics.
  • the ceramic electrode materials of the present invention are extremely compatible with the conventional ceramic insulating materials in chemical and physical properties.
  • the electrical devices formed with the ceramic electrode materials of the invention exhibit excellent electrical and electronic performance characteristics, in addition to improved reliability. Furthermore, the use of the ceramic electrode materials of the invention substantially reduces the material costs while also-easing manufacturing procedures for each device.

Abstract

Ceramic electrode materials (14, 16) of the invention include ceramic compositions having significant electrical conductivity at room temperature. The ceramic electrodes (14, 16) are coupled to a dielectric (12) to provide electrical devices such as capacitors(10) and transducers (10). The presently known ceramic compositions having room temperature conductivity are ceramic materials that are superconductors at low temperatures and include compositions of the formula: ZBa2-xAxO9-y, where Z is Y, La or any of the lanthanide series elements; A is Na, K, Rb, Cs or Pb; x is in the range 0.0 to 0.2; and y is about 2.0. Compositions having the formula: Bi2Sr3-xCaxCu2Oy, where x is in the range 0.0 to 2.0 and y is in the range 5.0 to 8.0 may also be used as ceramic electrodes (14, 16). The ceramic electrodes (14, 16) have similar perovskite crystalline structures to the dielectrics (12) forming the electrical devices (10) of the invention.

Description

CERAMIC ELECTRODE MATERIAL AND ELECTRICAL
DEVICES FORMED THEREWITH
This invention relates to ceramic material useful for forming electrodes in electrical devices and more particularly, to ceramic compositions being electrical conductors at ambient temperature.
Ceramic materials have been used for many years as insulators or dielectrics, having capactive, piezoelectric, ferroelectric and electrostrictive properties. The ceramic insulators are used in the fabrication of electrical devices, such as, capacitors, transducers and actuators. The devices are generally manufactured by depositing metal electrodes in the form of a paste or ink on opposing sides of the ceramic insulating material and firing the composite to complete the device.
Over the past two decades, the use of ceramics has grown with the introduction of multilayer ceramic devices. In the multilayer type, alternate layers of ceramic insulating and electrode materials are formed by a sequential or a lamination technique. Generally, in order to achieve high volume efficiency in the performance of the devices, the number of layers is increased. The metal electrodes which are generally selected from a group of precious metals based systems, can represent as much as 75% of the cost of the multilayer devices. Moreover, the devices are often fired at temperatures of 900°C or above which results in delamination problems between the precious metal electrodes and the insulating material. If non-precious metals .are utilized, a
special reducing atmosphere must be maintained during firing which may cause a change in the chemistry of the insulating material adversely affecting the performance of the device.
Recently, proposals have been made to overcome the problems associated with metal electrodes. In U.S. Patent 4,663,826, the surface region of a ceramic dielectric is irradiated with a laser beam in a low oxygen atmosphere to sinter the region, which causes the region to become conductive. In addition to requiring special atmospheric conditions, problems may result with the stability and the amount of conductivity of the resulting device.
The discovery of the BaLaCuO system of ceramic material exhibiting superconductivity at critical temperatures above 50°K, has heightened the interest in the use of ceramics as electrical conductors. Ceramic materials have been used in the prior art as electrodes in liquid crystal displays to apply an electric field to the liquid crystals. Other uses for ceramics are as ionic conductors in high temperature fuel cells and in electrochlorination systems.
U.S. Patent 4,316,785 is directed to a Josephson Junction utilizing a superconducting oxide material of BaPb1-xBixO3 as thin film electrode layers sputtered onto an oxide barrier layer. During operation, the barrier layer acts like a semiconductor to allow charge carriers to tunnel through the layer. The electrode and barrier layers both have perovskite crystalline structures, favorably affecting the stability of the device. The Josephson junction tunneling effect only occurs in materials that are
superconducting and therefore the oxide material disclosed is useful as an electrode only at very low temperatures, such as 13°K. There is no teaching of a ceramic material being an electrical conductor at room temperature for connection to a ceramic insulator for forming a single or multilayer electrical device.
The present invention is directed to ceramic compositions that are electrical conductors at ambient temperatures for forming ceramic electrodes for connection to a ceramic insulator or dielectric. The ceramic electrodes are electrically coupled to the ceramic insulator to produce electrical devices such as single and multilayer capacitors, piezoelectric transducers and electrostriction devices.
The ceramic compositions useful for forming the room temperature electrodes of the invention are ceramics exhibiting superconductivity at low temperatures. The ceramic superconductor materials especially useful as electrodes for electrical devices are those having a critical temperature above 50°K. Although the ceramic electrode materials are superconductors, the material need not be superconducting for the electrical devices to operate properly in their intended purpose. The ceramic electrodes of the invention have sufficient conductivity above the superconducting critical temperature to function effectively as electrodes at room temperature.
The superconductor ceramic materials that are useful as electrodes may be selected from the YBaCuO system and the LaBaCuO system. In addition, the electrodes may be made from materials of the BiSrCaCuO system and the TIBaCaCuO system. Ceramic compositions having higher critical temperatures are being discovered almost daily and any such composition may be utilized in the present invention. An example of the particular phases of the high Tc ceramic superconductor materials that are especially useful as electrodes are ceramics having the general formula ZBa2Cu3O9-y, where Z is Y, La or any of the other elements in the lanthanide series and where y is about 2. The elements in the lanthanide series are known as element numbers 58 through 71 of the periodic table of the elements. Another example is the high Tc superconductor ceramic composition having the formula Bi2Sr3-xCaxCu2Oy, where y is in the range from 5.0 to 9.0, and x is in the range from 0.0 to 1.0. In accordance with the present invention, the electrical devices of the invention comprise at least one layer of an electrical insulating ceramic composition and at least one layer of an electrical conducting ceramic composition electrically coupled to the ceramic insulating layer. The conducting ceramic composition is an electrical conductor at room temperature. By convention, an electrical conductor is a body so constructed that it may be used as a carrier of electric current. In ordinary usage, a conductor is a material of relatively high conductivity. An insulator is a material when placed between conductors at different potentials, permits only a small or negligible current to flow through it. The term dielectric is synonomous with electrical insulator. In ordinary usage, an insulator is a material of relatively high resistivity. Resistivity is inversely proportional to conductivity. The ceramic electrodes of the invention have a resistivity in the range of 100-500 μ ohms-cm at room temperature which is comparable to gold electrodes at ambient temperatures.
Many multilayer electrical devices such as capacitors, transducers, resonators and actuators are manufactured with a dielectric made from ceramic materials having a perovskite crystalline structure. For example, many multilayer capacitors have dielectrics based on barium, such as BaTiO3, which have a perovskite structure.
The ceramic electrodes of the present invention have a similar perovskite structure and are therefore more compatible with the ceramic dielectrics than the presently used metal electrode formulations. In addition, the ceramic electrodes are substantially less costly than the metal electrodes. The ceramic electrodes of the invention and insulating materials forming the electrical devices of the present invention have similar thermal expansion, latticeconstant and other related chemical properties eliminating existing processing problems of thermal expansion mismatch and delamination in the manufacture of multilayer electrical devices. It is well known that device performance is highly dependent on the integrity of the insulator, since any voids or defects in the insulator will significantly reduce efficiency and reliability of the device. Thus, the electrical devices of the present invention have an overall increase in device reliability from. the use of the ceramic electrode materials disclosed herein. In addition, the oxidation firing conditions for the ceramic electrode materials are similar to the ceramic dielectrics allowing for coprocessing of the electrodes and the insulating materials which will improve device reliability by enhancing defect chemistry control and also having the potential to reduce manufacturing costs.
The compatibility between the layers of the devices is optimized by combining the ceramic superconductor compositions containing barium with the barium based dielectrics. Similarly, the ceramic superconductor compositions containing bismuth are more compatible with bismuth based dielectrics. Moreover, to further enhance compatibility, the inventors have discovered that the barium containing ceramic electrode compositions may be modified by replacing a small percentage of the barium with one of a group of other elements to provide new superconducting compositions. The novel ceramic compositions have the formula ZBa2-xAxCu3O9-y where Z is Y, La or any of the other lanthanide series elements; A is Na, K, Rb, Cs, or Pb; x is in the range of 0.01 to 0.2; and y is about 2. The new compositions are especially useful as electrodes in the single or multilayer electrical devices described above which have insulating materials containing Pb, Na, K, Rb or Cs. The element substituted for Ba in the electrode is selected to match the element contained in the insulator to provide further compatibility between the layers of the device.
Figure 1 is a side sectional view of a ceramic electrical device of the present invention.
Figure 2 is a side sectional view of a multilayer electrical device of the present invention.
Figure 3 is a graph of dielectric constant versus temperature for one embodiment of a multilayer capacitor of the present invention.
Figures 4a and 4b are graphs of resistance versus temperature for one superconducting composition of the present invention before (4a) and after (4b) exposure to an oxidizing environment.
The ceramic electrode materials of the present invention are electrical conductors at room temperature and are useful as electrodes for single and multilayer electrical devices. The ceramic electrodes are coupled to a ceramic insulating material to form capacitors, transducers, actuators and the like. Presently, known ceramic compositions that are room temperature suitable as ceramic electrodes are high critical temperature, superconductor ceramic compositions. The ceramic superconductor materials having critical temperatures above 50°K have been found to have sufficient conductivity at ambient temperatures and a similar crystalline structure to the ceramic dielectric to produce electrical devices having excellent performance characteristics.
Representative high Tc ceramic compositions useful as electrode materials are: the ZBaCuO system where Z is Y,
La or any of the other elements in the lanthanide series; the layer structure BiSrCaCuO system and the TIBaCaCuO system.
In accordance with the invention, any high Tc ceramic superconducting composition may be used as the electrode material coupled to an insulating ceramic material to form an electrical device of the invention.
Referring now to the drawings, Figure 1 shows a single layer device 10 having a ceramic insulating body or dielectric 12 sandwiched between a pair of ceramic superconducting electrodes 14 and 16 electrically coupled to the insulating body 12. Figure 2 shows a multilayer electrical device 20 in which alternating layers of dielectric 22 and opposing electrodes 24 and 26 form the device. A pair of conductive terminations 28 and 30 provide the electrical connection between the electrodes 24 and 26 respectively. The devices 10 and 20 of Figures 1 and 2 may form a capacitor, or any other piezoelectric or electrostrictive device, such as, a resonator or an actuator. When devices 10 and 20 are ceramic capacitors, the opposing electrodes, 14 and 16 of device 10 and the opposing electrodes 24 and 26 of device 20, carry current to and from the subject device for storage, release or regulation of electrical energy. In a capacitor, the electrical energy is stored in the dielectric material and the relative ability of particular materials to store-.electrical energy is defined by its relative permittivity or dielectric constant. The capacitance is a function of the dielectric constant and capacitor performance is highly dependent upon the integrity of the dielectric.
The, current trend in dielectric materials, especially for capacitors and piezoelectric devices, is to utilize the higher permittivity titanates such as barium titanate. These ceramic dielectrics have a perovskite crystalline structure and require high sintering temperatures much higher than 950°C. In conventional processes for forming single and multilayer capacitors, after the dielectric is formed, the electrodes are formulated into a paste or ink and silk-screened or painted on the dielectric layers. Heretofore, capacitors having metal electrodes would suffer from delamination of the electrode layers due, in part, to the different sintering shrinkages between the metal and ceramic materials. In addition, during the manufacturing process, the interaction between the metal electrodes and ceramic dielectrics at the metal-ceramic interface would adversely alter the dielectric properties of the insulator. Moreover, in order to achieve high volume efficiency in multi-layer capacitors, the number of capacitor material layers has to be increased and hence the number of electrode layers must also be increased. In the present technology, the main cost of most of these devices comes from the precious metal electrode materials and can, in some devices, carry as much as 75% of the cost of the final component.
The ceramic compositions of the present invention have the same perovskite crystalline structure as the insulating material. The electrode and insulating material axe therefore compatible in chemistry, thermal expansion, lattice constant and other properties which facilitates processing flexibility and optimization and improves device performance. In addition, the use of the ceramic compositions of the present invention significantly reduces the material cost and the cost of production of the devices.
In fabricating the device, since the electrode materials are. ceramic, the oxidation firing conditions usually required for the multilayer processing are favorable with the ceramic superconductor electrodes. As a result, there are no special requirements to maintain a reducing atmosphere, as in the case with non-precious metal electrodes. Thus, device fabrication is facilitated as coprocessing of the electrodes and insulating materials can be easily performed.
In addition, the interfaces between the active insulating material and the superconductor electrodes are much more compatible. The delamination problem in multilayer capacitors will be substantially eliminated. In general, a slight modification of the superconductor composition during processing does not change the electrical properties of the interface of the composite devices. Dielectric defect chemistry is better controlled than in existing systems. The reduction in voids in the dielectric material after processing results in an increase in the overall. device reliability.
The superconductor ceramic materials can be prepared as electrodes having a resistivity of approximately 100 to 500μ ohms-cm at ambient temperatures. As the resistivity is inversely proportional to conductivity, the very low range of resistivity translates into a high range of .conductivity. Examples of ceramic superconductor compositions that can be used as electrode materials in forming electrical devices are ZBa2Cu3O9-y, where Z is Y, La or any of the lanthanide series elements and where y is about 2. Another ceramic superconductor composition suitable for electrode materials is Bi2Sr3-xCaxCu2Oy, where x is from 0.0 to 2.0 and y is from about 5.0 to about 8.0.
As an example, the preparation of one ceramic electrode composition of the invention begins with a mixture of Y2O3, BaO2 and CuO. The starting materials are mixed in the molar ratio as follows: ½Y2O3 + 2BaO2 + 3CuO. The mixture is ball milled and calcined at temperatures of about 920°C to 940°C for about 6 to 12 hours. The mixture is then pressed into pellet form and heated at a temperature of about 940°C to 960°C for about 6 to 12 hours. After oxidation, the resulting composition has the formula YBa2Cu3O7.
Figure 3 shows the measurements taken of the dielectric constant versus temperature, at 1KHz, of a multilayer capacitor built having electrodes of the formula YBa2Cu3O7 coupled to a BaTiO3 dielectric using conventional capacitor processing techniques. At room temperature, approximately 25°C., the dielectric constant is over 2000, which is similar to capacitors having gold electrodes. Also shown in Figure 3 is the dielectric loss, which is shown as about 0.01 at room temperature. Thus, dielectric integrity is maintained after processing.
Many of the multilayer capacitors and other devices are presently manufactured with lead based insulating materials or ferroelectrics. Examples of such ferroelectric materials are lead magnesium niobate, Pb (Mg1/3Nb2/3)O3 for electrostrictive devices and capacitors; lead iron niobate tungstate (Pb (FeNb)O3-Pb (FeW)O3 and lead zinc niobate Pb(ZnNb)O3 for piezoelectric, electrictive and capacitor applications and other materials such as, PbTiO3 and
Pb (Sc½Ta½)O3. In addition, many electrical devices have insulators containing alkali ions of Na, K, Rb or Cs. The inventors have determined that the ZBa2Cu3O9-y composition may be modified by replacing a small percentage of the Ba with one of the elements Pb, Na, K, Rb or Cs and that the resulting modified compositions have excellent room temperature conductivity properties while remaining superconductors at low temperatures. The inventors have further determined that by selecting the element for A to match the element upon which the insulator is based provides additional compatibility between the electrode and insulator layers, further enhancing device performance and reliability.
Thus, the novel superconductor composition has the general formula ZBa2-xAxCu3O9-y, where Z is Y, La or any lanthanide series elements; A is Pb, Na, K, Rb or Cs; -x is in the range 0.01 to 0.2; and y is about 2.
A sample was prepared utilizing 10% Pb replacement for the Ba in the composition of. the invention. In this example, therefore, x is equal to 0.2. In preparing the composition, the starting materials were mixed in the molar ratios 0.5Y2O3 + 1.8 BaO2 + 0.2 PbO + 3 CuO. The materials were ball milled and then calcined at 920°C for 10 hours. The mixture was then pressed into pellet form and heated slowly at 920°C for 6 hours. The pellets were then kept at 920°C for 6 hours. The pellets were then cooled slowly to 600°C at approximately 10°C per minute and kept at 600°C for 2 hours. The pellets were then cooled to room temperature at the rate of approximately 4°C per minute. The pellets had a composition of the formula YBa1.8Pb.2Cu3O9-y, where y was approximately 2. The pellets were then oxidized in an oxygen rich atmosphere at 600°C for 10 hours and cooled to room temperature to simulate the capacitor firing conditions. Resistance versus temperature measurements were taken and are shown in Figures 4a and 4b. Figure 4a shows the measurements before oxidation and Figure 4b shows the measurements after oxidation. In Figure 4a, at room temperature, about 300°K, the resistance was measured as about 8 μ ohms. In Figure 4b, the resistance at room temperature was about 7 μ ohms. Based on the dimensions of the sample, and using the known relationship between resistivity and resistance, defined by the formula
Figure imgf000014_0001
where P is the resistivity, R is the resistance, A is the area, and t is the thickness of the sample, the resistivity was calculated to be about 533^ ohms-cm and 466μ ohms per cm, respectively. In other samples, the resistivity can be lowered by a factor of 4-5. The range of resistivity of the electrodes of the invention is therefore from 100-500 μ ohms-cm. Thus, the ceramic electrodes have excellent room temperature conductivity both before and after oxidation. The inventors have determined that by adding a small amount, approximately 1%, of either K2CO3 or Rb2CO3 or CsCo3 with PbO during the initial preparation of the sample, helps densify the material without loosing any of the electrical conducting properties. The density is always greater than 90% of the theoretical density of the pure YBa2Cu3O9-y composition. The theoretical density of the pure material is 6.38 grams per cc.
Two samples were prepared of the BiSrCaCuO composition. The proper molar ratios of BiO, SrO, CaO and CuO were mixed and calcined at about 820°C. for 10 hours, into a fine powder. The powder was then pelletized and sintered at about 835°C. for about 6 hours. The pellets were then cooled and the resulting compositions had the formula Bi2Sr2CaCu2O8 and BiSrCa2Cu2O5.5. Similar resistance versus temperature tests were taken and the resistivity was also in the 100-500 μ ohm-cm range. A capacitor made with the electrodes formed from the BiSrCaCuO compositions coupled to a BaTiO3 dielectric has similar dielectric constant and loss characteristics as shown in Figure 3.
The ceramic electrode compositions of the present invention can be easily formulated into an ink or paste and applied to the insulator material during processing of the multilayer capacitors utilizing silk screening or painting operations similar to those currently used. A significant reduction in the number of steps necessary for producing multilayer capacitors is made possible by the present invention by the co-firing of the electrodes and the ceramic dielectric. Many of the dielectrics must be fired above 900°C. The ceramic material of the present invention reduces the number of preparation steps since the ceramic electrode can be fired at high temperatures with the dielectrics.
The ceramic electrode materials of the present invention are extremely compatible with the conventional ceramic insulating materials in chemical and physical properties. The electrical devices formed with the ceramic electrode materials of the invention exhibit excellent electrical and electronic performance characteristics, in addition to improved reliability. Furthermore, the use of the ceramic electrode materials of the invention substantially reduces the material costs while also-easing manufacturing procedures for each device.

Claims

WHAT IS CLAIMED IS:
1. A ceramic electrode for connection to a ceramic insulating body, said ceramic electrode comprising a ceramic composition being an electrical conductor at ambient temperature.
2. The ceramic electrode of claim 1 wherein the ceramic composition has a resistivity in the range between 100-500μ ohms-cm at ambient temperature.
3. The ceramic electrode of claim 2 wherein the ceramic composition is a superconductor.
4. The ceramic electrode of claim 3 wherein said superconductor ceramic composition has a critical temperature above 50°K.
5. The ceramic electrode of claim 4 wherein the ceramic composition is made from a system selected from the group containing LaBaCuO, YBaCuO, BiSrCaCuO and TIBaCaCuO.
6. The ceramic electrode of claim 5 wherein the ceramic composition has the formula ZBa2Cu3Og-y, where Z is Y, La or any of the lanthanide series elements and y is about 2.
7. The ceramic electrode of claim 5 wherein the ceramic composition has the formula ZBa2-xAxCu3O9-y where,
Z is Y, La or any of the lanthanide series elements,
A is Pb, Na, K, Rb or Cs, x is in the range from 0.01 to 2.0, and y is about 2.0.
8. The ceramic electrode of claim 5 wherein the superconductor ceramic composition has the formula Bi2Sr3-xCaxCu2Oy, where, x is in the range from 0.0 to 2.0 and y is in the range from 5.0 to 8.0.
9. An electrical device comprising: at least one layer of insulating ceramic material comprising a first ceramic composition being an electrical insulator; and at least one layer of conducting ceramic material electrically coupled to said at least one layer of insulating ceramic material, said at least one layer of conducting ceramic material comprising a second ceramic composition being an electrical conductor at ambient temperature.
10. The electrical device of claim 9 wherein the first and second ceramic compositions have substantially similar crystalline structures.
11. The electrical device of claim 10 wherein the first and second ceramic compositions have perovskite crystalline structures.
12. The electrical device of claim 11 wherein the second ceramic composition has a resistivity in the range between 100-500 μ ohms per cm at ambient temperature.
13. The electrical device of claim 12 wherein the second ceramic composition comprises a superconductor.
14. The electrical device of claim 13 wherein the second ceramic composition is made from a system selected from the group containing LaBaCuO, YBaCuO, BiSrCaCuO and TIBaCaCuO.
15. The electrical device of claim 14 wherein the second ceramic composition has the formula ZBa2-xAxCu3O9-y where ,
Z is Y, La or any of the lanthanide series elements,
A is Pb, Na, K, Rb or Cs. x is in the range 0.0 to 2.0, and y is about 2.0.
16. The electrical device of claim 14 wherein the second ceramic composition has the formula Bi2Sr3-xCaxCu2Oy, where, x is in the range 0.0 to 2.0, y is in the range 5.0 to 8.0.
17. The electrical device of claim 16 wherein the first ceramic composition contains Bi.
18. The electrical device of claim 16 wherein x is about 1.0 and y is about 8.0.
19. The electrical device of claim 16 wherein x is about 2.0 and y is about 5.5.
20. The electrical device of claim 15 wherein the fafrst ceramic composition contains Ba.
21. The electrical device of claim 20 wherein the first ceramic composition comprises BaTiO3.
22. The electrical device of claim 15 wherein the first ceramic composition contains Pb and wherein A of the second ceramic composition is Pb.
23. The electrical device of claim 22 wherein Z is Y and x is about 0.2.
24. The electrical device of claim 23 wherein the first ceramic composition comprises one of Pb (Mg1/3Nb2/3)O3, Pb(FeNb)O3-Pb(FeW)O3, Pb(ZnNb)O3, PbTiO3 and Pb(Sc½Ta½)O3.
25. The electrical device of claim 15 wherein the first ceramic composition contains one element selected from the group consisting of Na, K, Rb and Cs and wherein A of the second ceramic composition is the same as the selected element.
26. The electrical device of claims 9, 15 or 16 including a plurality of alternate layers of said insulating ceramic material and said conducting ceramic material thereby forming a multilayer electrical device.
27. The electrical device of claim 26 wherein said multilayer electrical device is a capacitor.
28. The electrical device of claim 26 wherein the multilayer electrical device is a piezoelectric device.
29. The electrical device of claim 26 wherein the multilayer device is an electrostrictive device.
30. The electrical device of claim 26 wherein the multilayer device is one of an actuator, transducer and resonator.
31. The electrical device of claim 26 wherein the first ceramic composition comprises one of BaTiO3, Pb(Mg1/3Nb2/3)O3, Pb (FeNb)O3-Pb(FeW)O3, Pb(ZnNb)O3, PbTiO3 and Pb(SC½Ta½)O3.
32. Compositions of the formula ZBa2-xAxCu3O9-y where:
Z is Y, La or any of the lanthanide series elements.
A is Na, K, Rb, Cs or Pb, x is 0.01 to 0.2, and y is about 2.
33. The compositions of claim 32 having perovskite crystalline structure.
34. The compositions of claim 33 being electrical conductors at ambient temperature.
35. The compositions of claim 34 being superconductors at critical temperatures of about. 90ºK or above.
36. The compositions of claim 35 wherein z is Y.
37. The compositions of claim 36 wherein x is about 0.2.
38. A process for preparing the compositions of claim 32 comprising:
(a) forming a mixture of Y2O3, BaO2, AO and CuO in effective amounts to provide the compositions of claim 32; (b) calcining the mixture at a temperature of about 920°C to 940°C for about 6 to 12 hours.
39. The process of claim 38 wherein the mixture of step (a) is formed with the mole ratios of 0.5Y2O3 + 1.8 BaO2 + 0.2 PbO + 3.0 CuO and wherein the composition is YBa1.8Pb0.2Cu3O7.
40. The process of claim 39 wherein the step of forming the mixture includes adding about 1% by weight of a compound selected from the group of K2CO3, Rb2CO3 and CsCO3 to the AO component.
PCT/US1989/002066 1988-05-13 1989-05-12 Ceramic electrode material and electrical devices formed therewith WO1989010813A1 (en)

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DE4416676A1 (en) * 1994-05-11 1995-11-23 Siemens Ag Device for the detoxification of exhaust gases from mobile systems
DE4416676C2 (en) * 1994-05-11 2002-11-07 Siemens Ag Device for the detoxification of exhaust gases from mobile systems
EP1333506A2 (en) * 2002-01-31 2003-08-06 Toyota Jidosha Kabushiki Kaisha Laminated piezoelectric actuator
EP1333506A3 (en) * 2002-01-31 2004-08-25 Toyota Jidosha Kabushiki Kaisha Laminated piezoelectric actuator
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WO2004020360A1 (en) * 2002-08-29 2004-03-11 Centre National De La Recherche Scientifique Ceramic with perovskite structure, use thereof as ph measuring electrode
WO2006068570A1 (en) * 2004-12-22 2006-06-29 Abb Research Ltd A method of manufacturing a varistor
US7525409B2 (en) 2004-12-22 2009-04-28 Abb Research Ltd. Method of manufacturing a varistor

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