WO2024056557A1

WO2024056557A1 - Ceramic materials including core-shell particles and varistors including the same

Info

Publication number: WO2024056557A1
Application number: PCT/EP2023/074810
Authority: WO
Inventors: Zumret Topcagic; Mirjam Cergolj
Original assignee: Ripd IP Development Ltd
Current assignee: Ripd IP Development Ltd
Priority date: 2022-09-14
Filing date: 2023-09-08
Publication date: 2024-03-21
Anticipated expiration: 2025-03-14

Abstract

Provided herein are varistor ceramic formulations that include a zinc oxide (ZnO)-coated particle having a core-shell structure, wherein the shell of the core-shell structure comprises ZnO, and the core of the core-shell structure has a specific resistance that is less than the specific resistance of the shell. Provided are varistor ceramic materials formed from such varistor ceramic formulations, and varistors formed from varistor ceramic materials of the invention. Also provided are methods of forming varistor ceramic materials.

Description

CERAMIC MATERIALS INCLUDING CORE-SHELL PARTICLES AND VARISTORS INCLUDING THE SAME

Field of the Invention

The present invention relates to zinc oxide-based ceramic materials. In particular, the present invention relates to zinc-oxide based ceramic materials that may be used in varistors for electrical devices.

Background of the Invention

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.

Traditionally, in addition to ZnO, 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.

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.

In some cases, 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.

Accordingly, there is a need for new materials and methods for producing varistor ceramic materials having lower protection voltages.

Summary of the Invention

According to some embodiments of the invention, provided are varistor ceramic formulations that include ZnO-coated particles having a core-shell structure. In some embodiments, the shell of the core-shell structure comprises 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). In such particles, 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. However, 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.

In some embodiments, 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. As an example, 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.

In some embodiments, 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. 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.

Also provided according to embodiments of the invention are 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.

Also provided according to some embodiments of the invention are methods of preparing a varistor ceramic material of the invention. In some embodiments, the methods include sintering a varistor ceramic formulation of the invention to produce the varistor ceramic material. In some embodiments of the invention, the methods further include the step of forming a ZnO-coated particle having a core-shell structure. For example, in some embodiments, 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. In some embodiments, 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.

Brief Description of the Drawings

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/cm²)) 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/cm²)) 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.

Detailed Description of Illustrative Embodiments

The present invention is now described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather these embodiments are provided so that this disclosure will be thorough and complete and will fully convey the scope of the invention to those skilled in the art.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms "a," "an" and "the" are intended to include plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" or "comprising," when used in this specification, specify the presence of stated features, integers, steps, operations, elements components and/or groups or combinations thereof, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components and/or groups or combinations thereof.

As used herein, the term "and/or" includes any and all possible combinations or one or more of the associated listed items, as well as the lack of combinations when interpreted in the alternative ("or").

Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly-used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the specification and claims and should not be interpreted in an idealized or overly formal sense unless expressly so defined herein. Well-known functions or constructions may not be described in detail for brevity and/or clarity.

It will be understood that when an element is referred to as being "on," "attached" to, "connected" to, "coupled" with, "contacting," etc., another element, it can be directly on, attached to, connected to, coupled with and/or contacting the other element or intervening elements can also be present. In contrast, when an element is referred to as being, for example, "directly on," "directly attached" to, "directly connected" to, "directly coupled" with or "directly contacting" another element, there are no intervening elements present. It will also be appreciated by those of skill in the art that references to a structure or feature that is disposed "adjacent" another feature can have portions that overlap or underlie the adjacent feature.

Spatially relative terms, such as "under," "below," "lower," "over," "upper" and the like, may be used herein for ease of description to describe an element's or feature's relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is inverted, elements described as "under" or "beneath" other elements or features would then be oriented "over" the other elements or features. Thus, the exemplary term "under" can encompass both an orientation of over and under. The device may otherwise be oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. Similarly, the terms "upwardly," "downwardly," "vertical," "horizontal" and the like are used herein for the purpose of explanation only, unless specifically indicated otherwise.

It will be understood that, although the terms 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. As used herein, 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. For example, “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.

As used herein, the term “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.

As used herein, the term “varistor ceramic formulation” refers to a composition that may be used and/or processed to form varistor ceramic material but has not yet been sintered.

As used herein, the term “granulate” refers to a varistor ceramic formulation that has been dried.

As used herein, the term “formed object” refers to a pressed granulate and may have any suitable shape or dimension. In some embodiments, the formed object is a disc or pellet.

As used herein, the term “varistor ceramic” or “varistor ceramic material” refers to a varistor ceramic formulation (or granulate or formed object) that has been sintered.

As used herein, the term “varistor” refers to a varistor ceramic material that has electrodes attached to or in electrical communication therewith.

In some embodiments of the invention, provided are 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). For example, 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. In some embodiments, 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.

Examples of substances that may be present in the core of the ZnO-coated particle, alone or in combination with other substances, include a metal doped-ZnO (e.g., Al-doped ZnO, Ga-doped ZnO, or In-doped ZnO) and/or indium tin oxide. 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. In particular embodiments, 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. In some embodiments, 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. In some embodiments, the ZnO is 100% pure, in some embodiments, greater than about 99.9% pure, and in some embodiments, greater than about 99.5% pure. In some embodiments, a small amount of impurities such as iron, copper, tin, and aluminum (e.g., up to 0.5%) may be present in the ZnO.

In some embodiments, 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. These compounds may also be referred to herein as “additional dopants.” Examples of such 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). Any suitable particle size of the additional dopants may be used. However, in some embodiments, 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. However, in some embodiments, 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 %);

Sb₂O₃, at a concentration in a range of about 0.01 mol % to about 2.0 mol %, in some embodiments, in a range of about 0.01, 0.1, 0.3, 0.4, 0.5, or 0.6 mol % to about 1, 1.2, 1.5, 1.6, 1.7, 1.8, 1.9, or 2.0 mol %, and, in particular embodiments, in a range of about 0.3 mol % to about 2.0 mol % (e.g., in a range of about 0.5 mol % to about 0.9 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

Cr₂O₃, at a concentration in a range of about 0 to about 1.0 mol %, and in some embodiments, in a range of about 0, 0.01, 0.02, 0.03, or 0.04 mol % to about 0.05, 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 to about 0.05 mol %.

In some embodiments, 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. In some embodiments, 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. Although any suitable solvent may be used, in some cases, water or an aqueous solution may be used as the solvent. Additionally, although many suitable 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. In general, 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.

Also provided according to embodiments of the invention are dried varistor ceramic formulations, also referred to as a granulate. In some embodiments, 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. Also provided are formed objects created by pressing the granulate. In some embodiments, the formed object is in the shape of a disk or pellet. In some embodiments, 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.

Also provided according to emboidments of the invention are varistor ceramic materials that may be formed from a varistor ceramic formulation of the invention. In some embodiments, 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). In some embodiments, the shell (e.g., ZnO) 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 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. In some embodiments, 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). In some embodiments, 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. Other substances, including other metal oxides, may be included in the core, as described above with respect to the ZnO-coated particles.

In some embodiments, 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.

In some embodiments, 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/cm³. Thus, in some embodiments, the density of the ceramic material is in a range of about 5.04 g/cm³ to about 5.6 g/cm³, and in some cases, in a range of about 5.26 g/cm³ to about 5.49 g/cm³.

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. As such, also provided herein are 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. However, in particular embodiments, the electrode is silver or aluminum. In some cases, 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. In some cases, varistor electrodes may include or consist of metallization layers on opposed outer surfaces of the varistor body.

In some embodiments, 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. In some embodiments, 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. In particular embodiments, 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, while 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. These waveforms are considered to approximate an indirect lightning strike and direct lightning strike, respectively. In some embodiments, 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. Moreover, 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.

In some cases, 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. Thus, 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.

In some embodiments of the invention, 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/cm² 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/cm² 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 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/cm² current density.

According to some embodiments of the invention, 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.

Also provided according to embodiments of the invention is an overvoltage protection device including a varistor ceramic material or varistor according to an embodiment of the invention. Provided according to some embodiments of the invention are 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). In some embodiments, the methods comprise sintering a varistor ceramic formulation of the invention to produce the varistor ceramic material.

Provided according to embodiments of the invention are methods of preparing 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. Each of these steps is discussed in further detail below.

In some embodiments, 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. In some embodiments, 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). In particular embodiments, 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.

In some embodiments, 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. In some embodiments, 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. In some cases, 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). In some embodiments, after pressing, the formed object 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. In some embodiments, 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. In some embodiments, after sintering the mixture, 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. In some embodiments, the electrode includes silver and is formed, for example, using a silver electrode screen printing process. As another example, 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. In some embodiments, 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.

In some embodiments, 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. After heating the formulation, 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. In some embodiments, 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. After heating the formulation, 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.

In some embodiments of the invention, 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. In some embodiments, a residual amount of metal (e.g., aluminum, gallium, and/or indium) ions may remain in the shell as impurities. In some embodiments, the methods further include forming the metal-doped ZnO particle by for example, bombarding a ZnO particle with a metal ion.

In some embodiments, 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. In some embodiments, the core particle has a specific resistance that is less than the specific resistance of the shell/coating. In some embodiments, 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.

Examples

Comparative Example 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.

Example 1

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. 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. 5, the effective grains resistance would be significantly lower than in conventional varistors while the grains outer ZnO layer establishes a suitable intergranular layer with conventional JE electrical characteristics in the nonlinear region. This behavior should produce a varistor which is the same as conventional varistors in the leakage and nonlinear regions while having improved characteristics in the surge region, thus producing lower residual voltages. Comparison of conventional varistor JE characteristics and the JE characteristics of expected with the varistors of the invention is illustrated in FIG. 6.

Example 2

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) may be formed by bombarding a ZnO particle with a metal ion such as an aluminum ion, a gallium ion, and/or an indium ion, thus forming a metal-doped ZnO. Then, 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.

While the present invention has been disclosed with reference to certain embodiments, numerous modifications to the described embodiments are possible without departing from the sphere and scope of the present invention, as defined in the following claims. Accordingly, it is intended that the present invention not be limited to the described embodiments, but that it has the full scope defined by the language in the claims.

Claims

That which is claimed is:

1. A varistor ceramic formulation comprising: a zinc oxide (ZnO)-coated particle having a core-shell structure, wherein the shell of the core-shell structure comprises 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 the ZnO).

2. The varistor ceramic formulation of claim 1, wherein the shell has a specific resistance that is at least 2, 10, 100, 1000, or 10,000 times greater than the specific resistance of the core.

3. The varistor ceramic formulation of claim 1 or 2, wherein the core has a specific resistance, and/or comprises a material having a specific resistance, in a range of about 1 10'⁵ Qcm to about 10 Qcm.

4. The varistor ceramic formulation of any one of claims 1-3, wherein the core comprises a metal-doped ZnO (e.g., Al-doped ZnO, Ga-doped ZnO, and/or In-doped ZnO) and/or indium tin oxide, optionally wherein the core comprises ZnO doped with less than about 1% by weight of metal.

5. The varistor ceramic formulation of any one of claims 1-4, wherein the formulation further includes one or more additional dopant particles, optionally wherein the formulation further comprises bismuth(III) oxide (ffeCh), antimony(III) oxide (Sb20s), cobalt tertraoxide (CO3O4), trimanganese tetraoxide (MnsO4), nickel(II) oxide (NiO), and/or chromium(III) oxide (CnCh).

6. The varistor ceramic formulation of any one of claims 1-5, wherein the formulation further comprises a binder and/or a plasticizer, optionally wherein the binder and/or the plasticizer are in total present in an amount of about 4% or less by weight of the formulation.

7. The varistor ceramic formulation of any one of claims 1 -6, wherein the formulation comprises a plurality of ZnO-coated particles having an average particle size in a range of about 0.1 pm to about 10 pm, optionally wherein the formulation comprises one or more additional dopant particles having an average particle size in a range of about 0.01 pm to about 10 pm.

8. The varistor ceramic formulation of any one of claims 1-7, wherein the shell of the ZnO-coated particle has a thickness that is about 1% to about 10% of the particle size of the particle.

9. A varistor ceramic formed by sintering the varistor ceramic formulation of any one of claims 1-8.

10. 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, wherein the shell of the core-shell structure comprises 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 the ZnO).

11. The varistor ceramic material of claim 10, wherein the shell has a specific resistance that is at least 2, 10, 100, 1000, or 10,000 times greater than the specific resistance of the core.

12. The varistor ceramic material of claim 10 or 11, wherein the core has a specific resistance, and/or comprises a material having a specific resistance, in a range of about 1 *10'⁵ Qcm to about 1 Qcm.

13. The varistor ceramic material of any one of claims 10-12, wherein the core comprises a metal-doped ZnO (e.g., Al-doped ZnO, Ga-doped ZnO, and/or In-doped ZnO) and/or indium tin oxide.

14. The varistor ceramic material of any one of claims 10-13, wherein the shell, and optionally the core, comprise at least one additional dopant particles (e.g., bismuth(III) oxide (Bi2Os), antimony(III) oxide (Sb20s), cobalt tertraoxide (CO3O4), trimanganese tetraoxide (MnsO4), nickel(II) oxide (NiO), and/or chromium(III) oxide (&2O3)).

15. The varistor ceramic material of any one of claims 10-14, wherein the one or more ceramic grain(s) have an average particle size in a range of about 2 pm to about 30 pm.

16. The varistor ceramic material of claim 15, wherein the shell of the core-shell structure of the one or more ceramic grain(s) has a thickness that is about 1% to about 10% of the diameter of the ceramic grains.

17. The varistor ceramic material of any one of claims 10-16, wherein the varistor ceramic material has a specific resistance in a range of about 2 Qcm to 10 Qcm measured at 0.8 kA (8/20 ps) and at 100 A/cm²; in a range of about 1-4 Qcm measured at 5 kA (8/20 ps) and at 625 A/cm²; and/or in a range of about 0.1-2 Qcm measured at 10 kA (8/20 ps) and at 1250 A/cm².

18. The varistor ceramic material of any of claims 10-17, wherein an intragranular boundary between ceramic grains comprises ZnO having a specific resistance in a range of about 1 Qcm to about 10 Qcm.

19. A varistor comprising: a varistor body comprising the varistor ceramic of any one of claims 10-18; and metallization layers on opposed outer surfaces of the varistor body.

20. The varistor of claim 19, wherein 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%.

21. The varistor of claim 19 or 20, wherein the varistor has a specific resistance in a range of about 2 to 10 Qcm measured at 0.8 kA (8/20 ps) and at 100 A/cm²; in a range of about 1-4 Qcm measured at 5 kA (8/20 ps) and at 625 A/cm²; and/or in a range of about 0.1-2 Qcm measured at 10 kA (8/20 ps) and at 1250 A/cm².

22. The varistor of any one of claims 19-21, wherein the varistor has a protection voltage (Up) in a range of about 200 V to about 265 V as measured at 5kA (8/20ps) and/or in a range of about 200 V to about 300 V as measured at lOkA (8/20ps).

23. The varistor of any one of claims 19-22, wherein the varistor has an energy absorption in a range of about 0.4 kJ/cm³ to about 0.9 kJ/cm³.

24. The varistor of any one of claims 19-23, wherein the varistor has a reduced voltage at current densities higher than about 625 A/cm2 relative to a reference varistor with a protection voltage to 1mA voltage ratio (Up/UlmA) of approximately two.

25. The varistor of any one of claims 19 - 23, wherein the varistor has a protection voltage to 1mA voltage ratio (Up/UlmA) of about 1.5 to about 1.6.

26. The varistor ceramic material of any of one of claims 10-18, or a varistor of any one of claims 19-25, wherein the core of the core-shell structure comprises ZnO doped with less than about 1% by weight of metal (e.g., aluminum).

27. An overvoltage protection device comprising the varistor of any one of claims 19-26.

28. A method of preparing a ceramic material, the method comprising: sintering the varistor ceramic formulation of any one of claims 1-8.

29. The method of claim 28, further comprising homogenizing the varistor ceramic formulation to provide a slurry before sintering, and optionally wherein the slurry is spray dried before sintering.

30. The method of claim 28 or 29, further comprising, prior to sintering the varistor ceramic formulation, pressing the varistor ceramic formulation into a formed object, and wherein sintering the varistor ceramic formulation comprises sintering the formed object.

31. The method of any one of claims 27-30, further comprising metallizing the ceramic material with at least one metal electrode.

32. The method of any one of claims 27-31, wherein sintering the formulation comprises heating the formulation to a temperature in a range of about 1,100 °C to about 1,300 °C, and cooling the heated formulation, to form the varistor ceramic material, optionally wherein the formulation has been homogenized, spray-dried, and/or pressed into a formed object before the heating of the formulation.

33. The method of any one of claims 27-32, further comprising the step of forming the ZnO-coated particle, wherein core of the core-shell structure comprises metal-doped ZnO (e.g., aluminum-doped ZnO), wherein forming the ZnO-coated particle comprises: providing a metal-doped ZnO particle, optionally wherein the metal-doped ZnO particle has a specific resistance in a range of about 1 *10'⁵ Qcm to about 10 Qcm; and removing metal (e.g., aluminum) ions from an outer layer of the metal-doped ZnO particle to form the shell comprising ZnO, thereby forming the ZnO-coated particle.

34. The method of any one of claims 27-32, further comprising the step of forming the ZnO-coated particle, wherein forming the ZnO-coated particle comprises: providing a core particle having a specific resistance that is less than ZnO (e.g., in a range of about 1 x 10'⁵ Qcm to about 10 Qcm); and coating the core particle with a composition comprising ZnO to form the shell of the core-shell structure, wherein the core particle forms the core of the core-shell structure.

35. The method of claim 34, wherein the core particle comprises a metal-doped ZnO or indium tin oxide.