WO2024056557A1 - Matériaux céramiques comprenant des particules cœur-écorce et varistances les comprenant - Google Patents

Matériaux céramiques comprenant des particules cœur-écorce et varistances les comprenant Download PDF

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WO2024056557A1
WO2024056557A1 PCT/EP2023/074810 EP2023074810W WO2024056557A1 WO 2024056557 A1 WO2024056557 A1 WO 2024056557A1 EP 2023074810 W EP2023074810 W EP 2023074810W WO 2024056557 A1 WO2024056557 A1 WO 2024056557A1
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varistor
core
zno
range
formulation
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Zumret Topcagic
Mirjam Cergolj
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Ripd Ip Development Ltd
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    • C04B35/626—Preparing or treating the powders individually or as batches ; preparing or treating macroscopic reinforcing agents for ceramic products, e.g. fibres; mechanical aspects section B
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    • C04B35/453—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 zinc, tin, or bismuth oxides or solid solutions thereof with other oxides, e.g. zincates, stannates or bismuthates
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    • C04B35/457—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 zinc, tin, or bismuth oxides or solid solutions thereof with other oxides, e.g. zincates, stannates or bismuthates based on tin oxides or stannates
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    • H—ELECTRICITY
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    • H01C—RESISTORS
    • H01C17/00—Apparatus or processes specially adapted for manufacturing resistors
    • H01C17/06—Apparatus or processes specially adapted for manufacturing resistors adapted for coating resistive material on a base
    • H01C17/065—Apparatus or processes specially adapted for manufacturing resistors adapted for coating resistive material on a base by thick film techniques, e.g. serigraphy
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    • H01C17/06513—Precursor compositions therefor, e.g. pastes, inks, glass frits characterised by the resistive component
    • H01C17/06533—Precursor compositions therefor, e.g. pastes, inks, glass frits characterised by the resistive component composed of oxides
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    • H01—ELECTRIC ELEMENTS
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    • H01C7/00—Non-adjustable resistors formed as one or more layers or coatings; Non-adjustable resistors made from powdered conducting material or powdered semi-conducting material with or without insulating material
    • H01C7/10—Non-adjustable resistors formed as one or more layers or coatings; Non-adjustable resistors made from powdered conducting material or powdered semi-conducting material with or without insulating material voltage responsive, i.e. varistors
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Definitions

  • the present invention relates to zinc oxide-based ceramic materials.
  • the present invention relates to zinc-oxide based ceramic materials that may be used in varistors for electrical devices.
  • Zinc oxide (ZnO) varistors are multicomponent ceramic devices with nonlinear currentvoltage (I-U) characteristics and high current and energy absorption capabilities. Due to these unique and desirable physical properties and a relatively cost-effective production, ZnO varistors have been used for the protection of electronic components against voltage surges. They have also been used for voltage stabilization over a broad range of voltages, from a few volts to several kilovolts.
  • varistors include dopants such as metal oxides (e.g., oxides of bismuth, antimony, cobalt, manganese, nickel, and chromium). Some of these dopants may create the nonohmic behavior in ZnO-based varistor ceramics while other dopants may enhance the nonlinear characteristics and help to control microstructure development.
  • dopants such as metal oxides (e.g., oxides of bismuth, antimony, cobalt, manganese, nickel, and chromium).
  • the microstructure of ZnO ceramic materials includes zinc oxide grains.
  • the bulk of the ZnO grain is highly conductive but the intergranular boundary, which may include the metal oxide dopants, may be highly resistive with nonlinear current-voltage (I-U) characteristics.
  • the threshold voltage (i.e., the breakdown voltage per unit thickness) of the varistor ceramics is generally directly proportional to the number of grain boundaries per unit of thickness and therefore is inversely proportional to the ZnO grain size.
  • the size of the ZnO grains depends on factors such as the chemical composition, firing temperature, and time.
  • Zinc oxide varistors limit voltage changes from transient currents to a certain level.
  • the varistors thus exhibit variable impedance, which depends either on the current flowing through the device or the voltage across the device’s terminals.
  • the voltage on the varistor during a transient current voltage disturbance may be called the protection voltage, residual voltage, or clamping voltage.
  • the protection voltage can also be defined as (1) the voltage at which the varistor eliminates the transient current’s disruption by connecting to ground or absorbing excess energy; or (2) the maximum voltage across the varistor before eliminating the transient current disturbance. When the varistor reaches the protection voltage, it blocks any further current through the device it protects by diverting the transient current to ground.
  • the protection voltage of a varistor it may be desirable to lower the protection voltage of a varistor so that more sensitive electrical devices are protected from even small voltage surges.
  • One possible method of lowering the protection voltage may be to decrease the post-sintering grain resistance of the varistor ceramic material. Reducing the post-sintering grain resistance may be achieved by increasing the amount of dopants in the ZnO starting material but this approach may also significantly affect the nonlinear electrical behavior of the intergranular layer. Therefore, this approach may not be ideal.
  • varistor ceramic formulations that include ZnO-coated particles having a core-shell structure.
  • the shell of the core-shell structure comprises ZnO
  • the core of the core-shell structure has a specific resistance that is less than the specific resistance of the shell (e.g., less than the specific resistance of ZnO).
  • the outer layer may have the same chemical properties as ZnO in conventional varistors and thus may form a suitable intergranular layer with standard varistor formulations and/or methods.
  • the lower specific resistance, p g (Qcm) of the core may provide a lower residual voltage in the resulting varistor ceramic material relative to conventional varistors formulations.
  • the core of the core-shell structure of the ZnO-coated particle has a specific resistance, and/or comprises a material having a specific resistance, in a range of about 1 xIO' 5 Qcm to about 10 Qcm.
  • the core of the core-shell structure may include a metal doped- ZnO and/or indium tin oxide. Other materials, including other metal oxides, may also be included in the core of the core-shell structure of the ZnO-coated particle.
  • the varistor ceramic formulations may also include additional dopant particles (e.g., other metal oxides particles) that may enhance varistor properties. Further, varistor ceramic formulations may optionally include additional components such as a solvent, binder, and/or plasticizer. Also provided according to some embodiments of the invention are varistor ceramic materials including those formed by a varistor ceramic formulation of the invention. In some embodiments, the varistor ceramic material includes an aggregate of ceramic grains, wherein one or more ceramic grain(s) (e.g., a plurality of ceramic grains) in the aggregate has a core-shell structure.
  • additional dopant particles e.g., other metal oxides particles
  • varistor ceramic formulations may optionally include additional components such as a solvent, binder, and/or plasticizer.
  • varistor ceramic materials including those formed by a varistor ceramic formulation of the invention.
  • the varistor ceramic material includes an aggregate of ceramic grains, wherein one or more ceramic grain(s) (e.g., a plurality of ceramic grains) in the aggregate has a core-
  • the shell of the core-shell structure includes ZnO, and the core of the core-shell structure may have a specific resistance that is less than the specific resistance of the shell (e.g., less than the specific resistance of ZnO). In some embodiments, the shell has a specific resistance that is at least about 2, 10, 100, 1000, or 10,000 times greater than the specific resistance of the core and/or a substance therein. In some embodiments, the core of the ceramic grains has a specific resistance in a range of about 1 * 10' 5 Qcm to about 1 Qcm. In some embodiments, the core of the ceramic grains includes a metal doped-ZnO and/or indium tin oxide. In some embodiments, the aggregate of ceramic grains further includes one or more additional dopants.
  • varistors that include a varistor ceramic material of the invention and metal electrodes on and/or in electrical connection to the varistor ceramic material. Further provided are overprotection devices including a varistor of the invention.
  • the methods include sintering a varistor ceramic formulation of the invention to produce the varistor ceramic material.
  • the methods further include the step of forming a ZnO-coated particle having a core-shell structure.
  • forming a ZnO-coated particle having a core-shell structure includes providing a metal-doped ZnO particle; and removing metal ions from an outer layer of the metal-doped ZnO particle to form the shell comprising ZnO, thereby forming the ZnO-coated particle.
  • forming the ZnO-coated particle includes providing a core particle; and coating the core particle with a composition comprising ZnO to form a shell on the core particle.
  • FIG. 1 is a flow chart providing steps in a conventional process of forming ZnO-based varistors.
  • FIG. 2 is a simplified illustration of post-sintered varistor ceramic microstructure of a conventional ZnO-based varistor.
  • FIG. 3 is a graph showing typical JE characteristics (E (V/mm) vs. J (A/cm 2 )) of a conventional ZnO varistor.
  • FIG. 4 is a flow chart providing steps in a method of forming of a varistor according to some embodiments of the invention.
  • FIG. 5 is a simplified illustration of an example of a post-sintered varistor ceramic microstructure of a varistor ceramic material according to an embodiment of the invention.
  • FIG. 6 is a graph showing the typical JE characteristics (E (V/mm) vs. J (A/cm 2 )) of a conventional metal oxide varistor compared to that expected for a varistor according to an embodiment of the invention.
  • FIG. 7 is an illustration of a method of forming a ZnO-coated particle according to an embodiment of the invention.
  • the device may otherwise be oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly.
  • the terms “upwardly,” “downwardly,” “vertical,” “horizontal” and the like are used herein for the purpose of explanation only, unless specifically indicated otherwise.
  • first, second, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. Rather, these terms are only used to distinguish one element, component, region, layer and/or section, from another element, component, region, layer and/or section. Thus, a first element, component, region, layer or section discussed herein could be termed a second element, component, region, layer or section without departing from the teachings of the present invention.
  • the sequence of operations (or steps) is not limited to the order presented in the claims or figures unless specifically indicated otherwise.
  • the term “about,” when referring to a measurable value, such as an amount or concentration or the like, is meant to refer to a variation of up to ⁇ 20% of the specified value, such as but not limited to, ⁇ 10%, ⁇ 5%, ⁇ 1%, ⁇ 0.5%, or ⁇ 0.1 of the specified value and including the specified value.
  • “about X” where X is a measurable value is meant to include X as well as variations of ⁇ 20%, ⁇ 10%, ⁇ 5%, ⁇ 1%, ⁇ 0.5%, or ⁇ 0.1 of X.
  • total metal and/or metalloid compounds includes all metal and metalloid compounds in a varistor ceramic formulation. As such, in some embodiments, it includes all components of a varistor ceramic formulation except for the solvent, plasticizer, and/or binder.
  • variable ceramic formulation refers to a composition that may be used and/or processed to form varistor ceramic material but has not yet been sintered.
  • the term “granulate” refers to a varistor ceramic formulation that has been dried.
  • the term “formed object” refers to a pressed granulate and may have any suitable shape or dimension.
  • the formed object is a disc or pellet.
  • variable ceramic or “varistor ceramic material” refers to a varistor ceramic formulation (or granulate or formed object) that has been sintered.
  • varistor refers to a varistor ceramic material that has electrodes attached to or in electrical communication therewith.
  • varistor ceramic formulations that include a ZnO-coated particle (or a plurality of ZnO-coated particles) having a core-shell structure.
  • the shell of the core-shell structure includes ZnO, and the core of the core-shell structure has a specific resistance that is less than the specific resistance of the shell (e.g., less than the specific resistance of ZnO).
  • the shell has a specific resistance that is at least about 2, 10, 100, 1000, or 10,000 times greater than the specific resistance of the core.
  • the core of the core-shell structure has a specific resistance, and/or comprises of a substance having a specific resistance, in a range of about I xlO' 5 Qcm to about 10 Qcm prior to sintering and a specific resistance in a range of about .001 Qcm to about 1 Qcm post sintering.
  • substances that may be present in the core of the ZnO-coated particle include a metal doped-ZnO (e.g., Al-doped ZnO, Ga-doped ZnO, or In-doped ZnO) and/or indium tin oxide.
  • a metal doped-ZnO e.g., Al-doped ZnO, Ga-doped ZnO, or In-doped ZnO
  • Other metal oxides may also be included in the core, alone or in combination with other substances.
  • the substance in the core may be varied but, in general, is a material that has a specific resistance that is lower, and in some embodiments, significantly lower than ZnO (and/or the shell) and is compatible with the coating and/or sintering process.
  • the core of the core-shell structure comprises, consists essentially of, or consists of ZnO doped with aluminum, gallium, and or indium at less than about 1% by weight (e.g., about 0.1 to about 1% by weight Al, Ga, and/or In).
  • the shell of the core-shell structure of the ZnO-coated particle comprises ZnO.
  • the ZnO has a specific resistance in a range of about 1 Qcm to about 10 Qcm post sintering.
  • the shell of the core-shell structure is generally continuous about the particle but in some embodiments, a portion of one or more ZnO-coated particle(s) does not include a shell and/or the core is partially and/or discontinuously coated.
  • the ZnO is 100% pure, in some embodiments, greater than about 99.9% pure, and in some embodiments, greater than about 99.5% pure.
  • a small amount of impurities such as iron, copper, tin, and aluminum (e.g., up to 0.5%) may be present in the ZnO.
  • the varistor ceramic formulations may further include other compounds (e.g., other metal oxides) as additional dopants that may enhance the varistor properties, such as, for example, by altering the properties of the intergranular boundaries between the core-shell grains in the resulting varistor ceramic material.
  • additional dopants include, but are not limited to, bismuth (III) oxide (ffeCh), antimony (III) oxide (Sb20s), cobalt tertraoxide (CO3O4), trimanganese tetraoxide (MmO-i), nickel (II) oxide (NiO), and chromium (III) oxide (CT2O3).
  • the average particle size of the additional dopant particles in the varistor ceramic formulation is in a range of about 0.01 pm to about 10 pm. In some embodiments, the average particle size of the additional dopant particles is in a range of about 0.01, 0.1, 0.2, 0.5, 1, 1.5, 2, 2.5, or 3 pm to about 7, 7.5, 8, 8.5, 9, 9.5, or 10 pm.
  • the additional dopants may be present in the varistor ceramic formulations in any suitable concentration.
  • the following compounds may be present (separately or in any combination thereof) in the following concentration ranges, each based on the total metal and/or metalloid compounds in the formulation:
  • Bi20s at a concentration in a range of about 0.1 mol % to about 1.5 mol %, in some embodiments, in a range of about 0.1, 0.2, 0.3, 0.4, or 0.5 mol % to about 1.0, 1.1, 1.2, 1.3, 1.4 or 1.5 mol %, and in particular embodiments, in a range of about 0.1 mol % to about 1.3 mol % (e.g., in a range of about 0.4 mol % to about 0.8 mol %);
  • CO3O4 at a concentration in a range of about 0.01 mol % to about 1.0 mol %, and in some embodiments, in a range of about 0.01, 0.05, or 0.1 to about 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, or 1.0 mol %, and in particular embodiments, in a range of about 0.01 mol % to about 0.3 mol%;
  • M C at a concentration in a range of about 0.01 mol % to about 1.0 mol %, and in some embodiments, in a range of about 0.01, 0.02, or 0.05 to about 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, or 1.0 mol %, and in particular embodiments, in a range of about 0.01 mol % to about 0.1 mol %;
  • NiO at a concentration in a range of about 0 mol % to about 1.0 mol %, and in some embodiments, in a range of about 0, 0.01, 0.05, or 0.1 mol % to about 0.2, 0.3, 0.4, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, or 1.0 mol %, and in particular embodiments, in a range of about 0.05 mol % to about 0.2 mol %; and/or
  • the varistor ceramic formulation includes a plurality of ZnO-coated particles, each having an average particle size of about 0.1 pm to about 10 pm.
  • the shell of the core-shell structure has a thickness that is about 1% to about 10% of the particle size of the ZnO-coated particle. It will be understood that the particles may not be completely spherical and may be oval or irregularly shaped.
  • a varistor ceramic formulation described above may further include a solvent, binder, and/or plasticizer prior to sintering.
  • a solvent may be used, in some cases, water or an aqueous solution may be used as the solvent.
  • binders and plasticizers are known in the art, in particular embodiments, polyvinylalchol (PVA) is included as a binder and polyethylene glycol (PEG) is included as a plasticizer.
  • a solvent may be present in the varistor ceramic formulation at a concentration in a range of about 20, 25, 30, or 25% to about 60, 65, 70, 75, or 80% by weight of the total formulation; and the binder and/or plasticizer together are present in the formulation at a concentration that is less than about 4 %, 3%, 2%, or 1% by weight (e.g., about 0.5, 1, 1.5, 2, 2.5, 3, 3.5, or 4% by weight, or any range defined therebetween), based on the total weight of the formulation.
  • the binder and/or plasticizer may be added to the solvent before the metal and/or metalloid compounds, with the metal and/or metalloid compounds, and/or after the metal and metalloid compounds in the formulation.
  • the granulate has an average particle size in a range of about 5 pm to about 300 pm, such as, for example, an average particle size in a range of about 5, 10, 15, 20, 25, 30, or 50 pm to about 250, 260, 270, 280, 290, or 300 pm.
  • formed objects created by pressing the granulate In some embodiments, the formed object is in the shape of a disk or pellet.
  • the formed objects have a green body density in a range of about 40 to about 70% of the theoretical sintered density of the body, including a green body density between about 40, 42, 45, or 50% to about 60, 62, 65, or 70% of the theoretical sintered density of the body.
  • varistor ceramic materials that may be formed from a varistor ceramic formulation of the invention.
  • the varistor ceramic formulation comprises an aggregate of ceramic grains, wherein one or more ceramic grain(s) (e.g., a plurality of ceramic grains) in the aggregate has a core-shell structure.
  • the shell of the core-shell structure includes ZnO, and the core of the core-shell structure may be or may include a substance that has a specific resistance that is less than the specific resistance of the shell (e.g., less than the specific resistance of the ZnO).
  • the shell e.g., ZnO
  • the shell has a specific resistance that is at least about 2, 10, 100, 1000, or 10,000 times greater than the specific resistance of the core and/or a substance therein.
  • the core of the ceramic grain(s) has a specific resistance, or comprises a substance having a specific resistance, in a range of about I xlO' 5 Qcm to about 1 Qcm.
  • the ZnO shell of the ceramic grains(s) has a specific resistance greater than about 1 Qcm post sintering (e.g., about 1-10 Qcm).
  • the core of the ceramic grains includes a metal doped-ZnO (e.g., Al-doped ZnO, Ga-doped ZnO, or Indoped ZnO) and/or indium tin oxide.
  • a metal doped-ZnO e.g., Al-doped ZnO, Ga-doped ZnO, or Indoped ZnO
  • indium tin oxide e.g., Al-doped ZnO, Ga-doped ZnO, or Indoped ZnO
  • Other substances, including other metal oxides, may be included in the core, as described above with respect to the ZnO-coated particles.
  • the aggregate of ceramic grains further includes one or more additional dopants therein.
  • the dopants are typically concentrated in the intergranular region and/or the shell of the ceramic grains, but in some embodiments, a portion of the dopants may be present in the core of the ceramic grains as well.
  • Dopants commonly used in varistor materials may be used, including, e.g., bismuth(III) oxide (ffeCh), antimony(III) oxide (Sb20s), cobalt tertraoxide (CO3O4), trimanganese tetraoxide (M C ), nickel(II) oxide (NiO), and/or chromium(III) oxide (CnCh), as described above with respect to the ZnO-coated particles.
  • the size and/or diameter of the ceramic grains may be varied. In some embodiments, however, the ceramic grains have an average particle size in a range of about 2 pm to about 30 pm. In some embodiments, the ceramic grains have an average particle size in a range of from about 2, 3, 4, 5, or 10 pm to about 20, 25, 28, 29 or 30 pm.
  • the thickness of the shell of the ceramic grains may also be varied, for example, in a range of about 1% to about 10% (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or any range defined therebetween) of the pariticle size of the grain.
  • the shell of the ceramic grain may or may not be continuous around each grain as, in some embodiments, particles may be sintered in such a way that a core region of a particle is adjacent to the intragranular boundary without a shell portion therebetween. In general, however, the shell region of the ceramic grains is present at the majority (at least 50%), substantially all (at least 90%, 95%, or 99%), or all of the intragranular boundaries.
  • the shape of the grain may vary depending on a number of factors, and in some cases, the shape of the ceramic grains may be irregular.
  • an intragranular boundary between ceramic grains comprises ZnO having a specific resistance in a range of about 1 Qcm to about 10 Qcm post sintering.
  • the properties of the varistor ceramic may vary depending on the composition, method of formation, the type of granulate, and the like. However, in some embodiments, the varistor ceramic has a density in a range of about 90% to about 100% of the theoretical density (such as about 90, 92, or 94% to about 96, 97, 98, 99 or 100%), and in particular embodiments, in a range of about 94 % to about 98 % of the theoretical density.
  • the theoretical density of the ZnO-based varistor ceramics is approximately 5.6 g/cm 3 .
  • the density of the ceramic material is in a range of about 5.04 g/cm 3 to about 5.6 g/cm 3 , and in some cases, in a range of about 5.26 g/cm 3 to about 5.49 g/cm 3 .
  • the varistor ceramic materials formed by the methods described herein may have desirable electrical properties. Such properties may be ascertained by metallization of the formed objects.
  • varistors including a varistor ceramic material of the invention and metal electrodes on or in electrical connection to the varistor ceramic material.
  • the metallization process is known in the art and any suitable electrode material may be used.
  • the electrode is silver or aluminum.
  • the thickness of the electrode layer is in a range of about 1 pm to about 80 pm, including about 1, 2, 5, 10, or 30 pm to about 50, 60, 70, 75, or 80 pm.
  • varistor electrodes may include or consist of metallization layers on opposed outer surfaces of the varistor body.
  • varistors formed from the varistor ceramics according to the invention may have desirable stability profiles.
  • the stability may be measured by a variety of methods. Typically, the stability is tested by applying a maximum continuous operating voltage to the varistor under a certain condition, such as a specific temperature (e.g., 85 °C or 115 °C), and/or under a combination of temperature and humidity conditions (e.g., 40 °C/ relative humidity in a range of 95- 98%). The leakage current over time is then measured.
  • the varistor’s leakage current has a value not exceeding twice the initial value (and in some cases, a value not exceeding 1.5, 1.6, 1.7, 1.8, or 1.9 times the initial value) within the first 5 hours of the stability test. In some embodiments, the varistor’s leakage current decreases less than about 20 pA over at least about 480 hours when a maximum continuous operating voltage is applied at about 40 °C with a relative humidity in a range of about 95 to about 98%.
  • a varistor formed from a varistor ceramic according to the invention may have a protection voltage (Up) in a range of about 200 V to about 265 V as measured at 5kA (8/20ps), such as in a range of about 200, 205, 210, 215, or 220 V to about 250, 255, 260 or 265 V.
  • the varistor formed from a ceramic material of the invention may have a Up in a range of about 200 V to about 300 V as measured at lOkA (8/20ps), such as in a range of about 200, 205, 210, 215, or 220 V to about 280, 285, 290, 295, or 300 V.
  • the varistor may have a protection voltage (Up) of about 265 V or less (e.g., about 260, 255, 250, 240, 230, or 220 V or less) as measured at 5kA (8/20ps) and/or of about 300 V or less (e.g., about 295, 290, 280, 270 or 260 V or less) as measured at lOkA (8/20ps).
  • An 8/20 waveform produces a current surge that reaches a maximum value in 8 ps and decays to 50% of maximum current in 20 ps
  • an 10/350 waveform produces a current surge that reaches a maximum value (IMAX) in 10 ps and decays to 50% of maximum current in 350 ps.
  • IMAX maximum value
  • a varistor formed from a varistor ceramic according to the invention may have a protection voltage (Up) to 1mA voltage (UlmA) (voltage resulting from application of 1mA through the varistor) ratio Up/UlmA of less than 2 and, in some embodiments, in a range of about 1.5 - 1.6.
  • Conventional varistors may have a Up/UlmA ratio of about 2.
  • the varistor formed from a varistor ceramic according to the invention may have a reduced voltage at current densities higher than about 625 A/cm2 relative to a varistor with a Up/UlmA ratio of approximately two.
  • the energy absorption capability of a varistor according to an embodiment of the invention is in a range of about 0.4 kJ/cm3 to 0.5 kJ/cm3, but in some embodiments, may be as high as about 0.7 kJ/cm3 to about 0.9 kJ/cm3.
  • the energy absorption capability of the varistor may be in a range of about 0.4, 0.45, 0.5, or 0.55 kJ/ cm3 to about 0.7, 0.75, 0.8, 0.85, or 0.9 kJ/cm3.
  • the energy absorption of the varistor may also or alternatively comply with existing standards for surge protective devices including ZnO varistors, e.g., IEC/EN 61643-11, EN 50539-11, and/or IEC/EN 61643-31, which protocols are incorporated herein by reference.
  • ZnO varistors e.g., IEC/EN 61643-11, EN 50539-11, and/or IEC/EN 61643-31, which protocols are incorporated herein by reference.
  • a varistor formed from a ceramic material of the invention may have a specific resistance in a range of about 2 Qcm to about 10 Qcm measured at 0.8 kA (8/20ps) and with a 100 A/cm 2 current density. In some embodiments, a varistor formed from a ceramic material of the invention may have a specific resistance in a range of about 2 Qcm to about 4 Qcm (e.g., about 2.4 Qcm to about 3 Qcm) measured at 5 kA (8/20ps) with a 625 A/cm 2 current density.
  • a varistor formed from a ceramic material of the invention may have a specific resistance in a range of about 0.1 Qcm to about 2 Qcm (e.g., about 1.3 Qcm to about 1.7 Qcm) measured at 10 kA (8/20ps) with a 1250 A/cm 2 current density.
  • the protection voltage (Up) of a varistor may be influenced by the ohmic properties of varistor ceramics.
  • the current-voltage (I-U) characteristics of a varistor may include three primary areas: 1) pre-breakdown region (ohmic), which corresponds to voltages below the nominal voltage (UN) of the varistor where high resistance of varistor ceramics is determined by electrostatic barriers at the grain boundaries; 2) non-linear region above the nominal voltage (UN), which is characterized by the "fall” of electrostatic barriers and varistor ceramics turn into a high conductive state, so that current increases by several orders of magnitude with a light change in voltage; and 3) high currents region or upturn region, which is characterized by the varistor ceramics returning to have the nature of an ohmic resistor and its currentvoltage (I-U) characteristics are determined by the resistivity of the ZnO grains.
  • an overvoltage protection device including a varistor ceramic material or varistor according to an embodiment of the invention.
  • a varistor ceramic material or varistor according to an embodiment of the invention.
  • methods of preparing a varistor ceramic material comprising an aggregate of ceramic grains, wherein one or more ceramic grain(s) (e.g., a plurality of grains) in the aggregate has a core-shell structure (i.e., a varistor ceramic material according to an embodiment of the invention).
  • the methods comprise sintering a varistor ceramic formulation of the invention to produce the varistor ceramic material.
  • a varistor ceramic material that include mixing and homogenizing a varistor ceramic formulation according to an embodiment of the invention; drying the formulation and pressing it into a formed object such as a disk or pellet; and then sintering the formed object to provide the varistor ceramic material.
  • the varistor ceramic material may then be metallized to create a varistor.
  • a varistor ceramic formulation according to an embodiment of the invention is mixed and/or homogenized. If the varistor ceramic formulation initially does not include a solvent, binder, and/or plasticizer, the process may include mixing the ZnO-coated particles, and optionally additional metals or metalloids (e.g., dopant particles), with a solvent and, optionally, a binder and/or plasticizer, as described herein. In some embodiments, the ZnO-coated particles and any other metal or metalloid particles are first mixed with a solvent and then later a binder and/or plasticizer is added.
  • additional metals or metalloids e.g., dopant particles
  • all of the components may be mixed at one time, and in some embodiments, the binder and/or plasticizer may be added to the solvent before the metal and/or metalloid compounds are added.
  • the ZnO-coated particles and any additional metal or metalloaid compounds (and optionally, a binder and/or plasticizer) and a solvent as mixed may form a slurry.
  • the varistor slurry may then be mixed or homogenized for a suitable time, and in some cases for at least 20 hours (e.g., at least 20, 22, 24, 26, or 30 hours).
  • the materials are milled with a ball milling apparatus for a time in a range of 1 to 72 hours, such as for about 1, 2, 5, 10, 15, 20, or 25 hours to about 60, 62, 64, 66, 68, 70, or 72 hours.
  • the homogenized varistor ceramic formulation (e.g., a slurry) is spray dried to thereby form a granulate.
  • the drying processes for varistor compositions are known in the art, and any suitable method may be used, but in some embodiments, the varistor ceramic formulation is dried at a temperature in a range of about 110 °C to about 250 °C, such as a temperature in a range of about 110, 115, 120, 125, or 130 °C to about 225, 230, 235, 240, 245 or 250 °C.
  • the dried granulate has low water content (e.g., less than about 0.3 weight % of the granulate).
  • the granulate may be pressed to create a formed object such as a pellet or disk.
  • the diameter of the pellet or disk is in a range of about 5 mm to about 100 mm, such as in a range of about 5, 10, 15, 20, or 25 mm to about 80, 85, 90, 95, 100 mm, and the thickness is below about 15 mm (e.g., less than about 15, 12, 10, or 5 mm).
  • the formed object after pressing, has a green body density in a range of about 40% to about 70 % of the theoretical sintered density of the formed object, such as a green body density in a range of about 40, 42, 45, or 50% to about 60, 62, 65, or 70% of the theoretical sintered density of the formed object.
  • the formed object may be sintered to form a varistor ceramic of the invention.
  • the formed object may be sintered (e.g., in a kiln) at a temperature in the range of about 1100 °C to about 1300 °C (e.g., between about 1100, 1110, 1120, or 1150 °C to about 1250, 1260, 1270, 1280, 1290 to 1300 °C), and in particular embodiments, in a range of about 1150°C to about 1250°C, in an oxygen environment.
  • the formed object may be sintered for any suitable time but in some embodiments, the formed object is sintered for a time in a range of about 0.5 hours to about 10 hours (e.g., about 0.5, 1, 2, 3 to about 4, 5, 6, 7, 8, 9 or 10 hours), and in particular embodiments, in a range of about 1 hour to about 4 hours, to provide a ceramic material.
  • the ceramic material may be cooled to a temperature of about 850°C to about 1000°C, and in some embodiments, at a temperature in a range of about 850°C at a first cooling rate of at least about 10, 11, 12, 13, 14, or 15°C/min. In some embodiments, the ceramic material may then be cooled at a second cooling rate of less than about 3, 2 or 1 °C/min until the temperature is below about 200 °C.
  • the sintered ceramic material may be used to form a varistor by applying or attaching an electrode material, such as after the cooling process.
  • the electrode includes silver and is formed, for example, using a silver electrode screen printing process.
  • an aluminum electrode may be used.
  • the electrodes may have any suitable thickness, but in some embodiments, the electrode layer may have a thickness in a range of about 10 pm and about 20 pm, such as in a range of about 10, 11, 12, 13, or 15 pm to about 16, 17, 18, 19, or 20 pm.
  • the metallization process occurs using a firing temperature in a range of between about 500 °C to about 700°C, and for a time of approximately 5 minutes to 30 minutes, but any suitable method may be used. Subsequently, soldering of the metal electrode (typically on two opposing surfaces) and coating may be performed to prepare the varistor.
  • the formulation may be heated at a temperature in a range of about 1100°C to about 1200°C in an atmosphere including oxygen to produce the varistor ceramic material.
  • the ceramic material may be cooled to a temperature in a range of about 850°C to 1000 °C at a first cooling rate of at least about 15°C/min.
  • the ceramic material may then be cooled at a second cooling rate of less than about 3°C/min until a temperature of 200°C.
  • the formulation may be heated at a temperature in a range of about 1100°C to about 1200°C in an atmosphere including oxygen to produce the ceramic material.
  • the ceramic material may be cooled to a temperature of about 850°C at a first cooling rate of at least about 15°C/min. The ceramic material may then be cooled at a second cooling rate of less than about 3°C/min until a temperature of 200°C.
  • forming the ZnO-coated particle having a core-shell structure includes providing a metal-doped ZnO particle, optionally wherein the metal-doped ZnO particle has a specific resistance in a range of about 1 xlO' 5 Qcm to about 10 Qcm prior to sintering; and removing metal (e.g., aluminum, gallium, and/or indium) ions from an outer layer of the metal- doped ZnO particle to form the shell comprising ZnO, thereby forming the ZnO-coated particle.
  • a residual amount of metal (e.g., aluminum, gallium, and/or indium) ions may remain in the shell as impurities.
  • the methods further include forming the metal-doped ZnO particle by for example, bombarding a ZnO particle with a metal ion.
  • forming the ZnO-coated particle includes providing a core particle; and coating the core particle with a composition comprising ZnO to form a shell on the core particle.
  • the core particle has a specific resistance that is less than the specific resistance of the shell/coating.
  • the core particle has a specific resistance in a range of about 1 xlO' 5 Qcm to about 10 Qcm prior to sintering. Coating of the core particles may be achieved by a number of different methods, including, for example, vapor deposition.
  • FIG. 1 A flow chart illustrating a conventional ZnO varistor production method is shown in FIG. 1. Such methods result in varistor ceramics such as those illustrated in FIG. 2, which shows the creation of intergranular boundaries having nonlinear electrical characteristics and ZnO grains with purely ohmic electrical characteristics.
  • Typical varistor JE (current density vs. electric field) characteristics of such varistors is shown in FIG. 3, which shows particular operating regions: leakage, nonlinear and surge region. The surge region is generally controlled by the post-sintering grains specific resistance, thus defining the residual voltage level.
  • One proposed method is to use pre-sintered powder particles with a ZnO shell and a conductive AZO (Al-doped ZnO) core.
  • Such approach may provide the same outer chemical properties of powder mixture as in conventional varistors and thus it may provide suitable intergranular layer formation using known sintering methods and dopants.
  • the conductive cores of the AZO powder may provide low specific resistance pg (Qcm) in post-sintered ceramics and thus provide a lower residual voltage in a high-current region than conventional varistors.
  • Qcm specific resistance pg
  • FIG. 4 A flow chart of the proposed method is illustrated on FIG. 4. The method illustrated in FIG. 4 may result in post-sintering varistor ceramic materials similar to those illustrated in FIG. 5. As it can be seen from FIG.
  • a ZnO-coated particle may be formed by a number of different methods.
  • FIG. 7 illustrates one possible method.
  • a metal-doped ZnO particle e.g., AZO
  • AZO a metal ion such as an aluminum ion, a gallium ion, and/or an indium ion, thus forming a metal-doped ZnO.
  • the surface of the metal particle may be treated to deplete the metal ion concentration from a portion and/or outer layer of the particle.
  • Such a treatment may result in a core having a higher concentration of metal ions and a shell that is depleted of metal ions, resulting in a shell that is devoid of metal ions or has a reduced concentration of metal ions relative to the core of the particle.
  • the depletion of the ions in the shell of the particle creates a lower specific resistance in the core of the particle relative to that in the shell.

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Abstract

L'invention concerne des formulations céramiques de varistances qui comprennent des particules revêtue d'oxyde de zinc (ZnO) ayant une structure cœur-écorce, l'écorce de la structure cœur-écorce comprenant du ZnO, et le cœur de la structure cœur-écorce ayant une résistance spécifique qui est inférieure à la résistance spécifique de l'écorce. L'invention concerne des matériaux céramiques de varistances formés à partir de telles formulations céramiques de varistances, et des varistances formées à partir de matériaux céramiques de varistances de l'invention. L'invention concerne également des procédés de formation de matériaux céramiques de varistances.
PCT/EP2023/074810 2022-09-14 2023-09-08 Matériaux céramiques comprenant des particules cœur-écorce et varistances les comprenant WO2024056557A1 (fr)

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