Metal Oxide Varistor Compounds

Abstract

Provided is a metal oxide varistor comprising a sintered ceramic, the ceramic comprising about 91.0 wt % to about 97.0 wt % ZnO, at least 0.3 wt % Mn, at least 0.4 wt % Bi, at least 1.0 wt % Sb, and up to 0.50 wt % Co. The metal oxide varistors disclosed herein exhibit reduced power loss, improved thermal stability, and can be manufactured at a lower cost compared to conventional MOV devices.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of and priority to U.S. Provisional Application No. 62 / 951,183, filed December 20, 2019, the contents of which are incorporated herein by reference in their entirety and priority is claimed thereto.

[0002] The present disclosure relates to metal oxide varistor formulations that can be manufactured at lower cost and offer improved thermal stability compared to conventional formulations. [Background technology]

[0003] Surge protection in both high-voltage and low-voltage circuits is typically provided by surge arresters utilizing one or more metal oxide varistors (MOVs). MOVs function as voltage-dependent resistors that reversibly switch between low and high resistance over a narrow voltage range. Near the switching voltage, the relationship between voltage and current often follows a power law: I=CV α where α is the nonlinearity coefficient and C is a constant. Peak values ​​of α can exceed 40-50 depending on the material. MOVs are typically rated by the manufacturer for various characteristics, including maximum continuous operating voltage (MCOV), transient overvoltage (TOV) capability, and residual voltage during a specific surge event (e.g., a 10,000 A, 8 x 20 μs surge). Commercially available MOVs for high-voltage applications are rated to specific levels of load according to guidelines set forth in the US IEEE C62.11 standard or the international IEC 60099-4 standard.

[0004] The nonlinear VI behavior of MOVs is due to the doping elements. The microstructure of MOVs consists of n-type semiconducting ZnO grains and resistive grain boundaries. The grain boundaries are compositionally well-defined and contain Bi2O3 as well as modifier elements such as Mn, Co, and Sb. Interface states located at the grain boundaries act as electron traps, capturing conduction band electrons in the adjacent n-type ZnO. As a result, a double Schottky barrier is formed at the grain boundaries, blocking electron flow. Current leakage through the Schottky barrier is thermally activated and is 1.0-1.25 * The power dissipation in MOVs in the MCOV range is typically strongly temperature dependent. If the MOV overheats due to repeated exposure to surge loads, the power dissipated in the MCOV may exceed the level that can be dissipated in the arrester housing, causing the arrester to fail due to thermal runaway.

[0005] Therefore, there remains a need for alternative MOV formulations that improve thermal stability and nonlinear behavior. Summary of the Invention

[0006] In one aspect, the present disclosure provides: A sintered ceramic comprising: about 91.0 wt% to about 97.0 wt% ZnO; at least 0.3 wt.% Mn; at least 0.4 wt.% Bi; at least 1.0 wt. % Sb; and A metal oxide varistor is provided that includes a sintered ceramic containing 0.50 wt % Co or less.

[0007] In another aspect, the present disclosure provides a surge arrester for protecting one or more conductors against power surges, the surge arrester including a metal oxide varistor as described herein, wherein surge current is diverted from the conductor to ground through the metal oxide varistor.

[0008] The metal oxide varistors described herein exhibit reduced power loss, improved thermal stability, and can be manufactured at a lower cost compared to conventional MOVs.

[0009] Other aspects of the present invention will become apparent by consideration of the detailed description and accompanying drawings. [Brief explanation of the drawings]

[0010] [Figure 1] Figure 1 shows the W / kV MCOV as a function of temperature for the three MOV formulations provided in Table 1. The reference material ("REF") is a commercially available MOV used in IEEE C62.11-2012 "normal duty" surge arresters. DETAILED DESCRIPTION OF THE INVENTION

[0011] Before describing the embodiments herein in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or carried out in various ways.

[0012] As used herein, the terms "comprise," "include," "having," "has," "can," "contain," and variations thereof are intended to be open-ended transitional phrases, terms, or words that do not exclude the possibility of additional acts or structures. The singular forms "a," "an," and "the" include plural references unless the context clearly dictates otherwise. The present disclosure also contemplates other embodiments that "comprising," "consisting of," or "consisting essentially of" the embodiments or elements presented herein, whether expressly stated or not.

[0013] The modifier "about" used in connection with a quantity is inclusive of the stated value and has the meaning dictated by the context (e.g., includes at least the degree of error associated with measurement of the particular quantity). The modifier "about" should also be considered to disclose a range defined by the absolute values ​​of the two endpoints. For example, the phrase "about 2 to about 4" also discloses the range "2 to 4." In addition, "about" can refer to plus or minus 10% of the stated numerical value. For example, "about 10%" would indicate a range of 9% to 11%, and "about 1" could indicate a range of 0.9 to 1.1. Other meanings of "about" may be apparent from the context, such as rounding; for example, "about 1" could mean 0.5 to 1.4.

[0014] As used herein, when a range of numbers is given, each intervening number of equal precision is expressly contemplated. For example, in the range 6 to 9, the numbers 7 and 8 are contemplated in addition to 6 and 9, and in the range 6.0 to 7.0, 6.0, 6.1, 6.2, 6.3, 6.4, 6.5, 6.6, 6.7, 6.8, 6.9, and 7.0 are expressly contemplated.

[0015] "Metal oxide varistor" (MOV) refers to an oxide ceramic material, or electrical unit made therefrom, that exhibits a highly non-linear relationship between voltage and current, with the electrical resistivity of the material decreasing dramatically as the voltage is increased.

[0016] "Maximum Continuous Operating Voltage" (MCOV) is 0.4mA / cm at 20°C. 2 This refers to 80% of the reference voltage measured at peak current density (60 Hz). All voltages and currents are defined as 60 Hz peak values ​​unless otherwise specified. Voltage values ​​may also be displayed alternately in RMS units.

[0017] Unless otherwise specified, chemical elements as disclosed herein (e.g., Mn, Co, Bi, Sb) may be included in the formulations in any suitable molecular form, including, for example, commercially available oxides, carbonates, hydroxides, nitrates, mixed oxide precursors, and other molecular forms of the elements known in the art.

[0018] The present disclosure relates to chemical formulations for manufacturing MOVs that can be used in high voltage surge arresters, such as those that comply with IEEE C62.11, IEC 60099-4, or similar standards.

[0019] In one aspect, the present disclosure provides: A sintered ceramic comprising: about 91.0 wt% to about 97.0 wt% ZnO; at least 0.3 wt.% Mn; at least 0.4 wt.% Bi; at least 1.0 wt. % Sb; and A metal oxide varistor is provided that includes a sintered ceramic containing 0.50 wt % Co or less.

[0020] As used herein, sintered ceramic refers to a sintered composition that provides the ceramic body of a metal oxide varistor (MOV). It is understood that the sintered composition conforms to standards known in the art to meet the operating conditions of the MOV, such as temperature, voltage, and current. The metal oxide varistor may further include a first electrode having a first electrode surface and a second electrode having a second electrode surface. For example, the sintered ceramic may be sandwiched between two metal plates that function as electrodes of the varistor.

[0021] The formulation may comprise at least 91.0 wt%, at least 92.0 wt%, at least 93.0 wt%, at least 94.0 wt%, at least 95.0 wt%, or at least 96.0 wt% ZnO. The formulation may comprise less than 97.0 wt%, less than 96.0 wt%, less than 95.0 wt%, less than 94.0 wt%, less than 93.0 wt%, or less than 92.0 wt% ZnO. The formulation may comprise from about 92.0 wt% to about 97.0 wt%, from about 93.0 wt% to about 97.0 wt%, from about 94.0 wt% to about 97.0 wt%, or from about 95.0 wt% to about 97.0 wt% ZnO. In some embodiments, the formulation comprises from about 95.0 wt% to 97.0 wt%, e.g., about 95.5 wt%, about 96.0 wt%, or about 96.5 wt% ZnO.

[0022] The formulation may include 0.4 wt% or less, 0.3 wt% or less, 0.2 wt% or less, or 0.1 wt% or less cobalt (Co). In some embodiments, the formulation includes 0.20 wt% or less Co, such as 0.18 wt% or less, 0.15 wt% or less, 0.12 wt% or less, 0.10 wt% or less, 0.09 wt% or less, 0.08 wt% or less, 0.07 wt% or less, 0.06 wt% or less, 0.05 wt% or less, 0.04 wt% or less, 0.03 wt% or less, 0.02 wt% or less, 0.01 wt% or less, 0.005 wt% or less, or 0.001 wt% or less Co. In some embodiments, the formulation is Co-free.

[0023] Co is, at least in part, C o Cobalt oxide forms such as 3O4 may be included in the formulation. For example, the formulation may contain up to 0.68 wt.% C, which corresponds to up to 0.50 wt.% Co. o In some embodiments, the formulation may contain 0.50% by weight or less, 0.40% by weight or less, 0.30% by weight or less, 0.20% by weight or less, 0.10% by weight or less, 0.05% by weight or less, 0.01% by weight or less, 0.005% by weight or less, or 0.001% by weight or less C. o 3O4, etc., 0.60 wt% or less of C o In some embodiments, the formulation comprises C o Does not contain 3O4.

[0024] The formulation may contain at least 0.3 wt% manganese (Mn), such as at least 0.4 wt%, at least 0.5 wt%, at least 0.6 wt%, at least 0.7 wt%, at least 0.8 wt%, at least 0.9 wt%, or at least 1.0 wt% Mn. The formulation may contain less than 1.0 wt%, less than 0.9 wt%, less than 0.8 wt%, less than 0.7 wt%, less than 0.6 wt%, less than 0.5 wt%, or 0.4 wt% Mn. The formulation may contain at least 0.3 wt% to about 1.0 wt%, about 0.4 wt% to about 1.0 wt%, about 0.4 wt% to about 0.9 wt%, or about 0.4 wt% to about 0.8 wt% Mn. In some embodiments, the formulation includes about 0.4 wt% to about 0.8 wt% Mn, such as about 0.4 wt%, about 0.5 wt%, about 0.6 wt%, about 0.7 wt%, or about 0.8 wt% Mn.

[0025] Mn may be included in the formulation, at least partially, in the form of manganese oxides, such as MnO. For example, the formulation may include at least 0.39 wt% MnO, corresponding to at least 0.30 wt% Mn. In some embodiments, the formulation includes at least 0.40 wt% to about 1.30 wt%, about 0.50 wt% to about 1.30 wt%, about 0.50 wt% to about 1.20 wt%, about 0.50 wt% to about 1.10 wt%, or about 0.50 wt% to about 1.00 wt% MnO. In some embodiments, the formulation includes about 0.5 wt% to about 1.00 wt%, such as about 1.00 wt%, about 0.90 wt%, about 0.80 wt%, about 0.70 wt%, about 0.60 wt%, or about 0.50 wt% MnO.

[0026] In some embodiments, the total amount of Mn(x) and Co(y) in the formulation satisfies z = 1.29x + 1.36y, where x is at least 0.31%, y is 0.00% to 0.50%, and z is about 0.75% to about 1.25%. The value of z can be about 0.80%, about 0.90%, about 1.00%, about 1.10%, or about 1.20%.

[0027] The formulation may contain at least 1.0 wt% antimony (Sb), such as at least 1.25 wt%, at least 1.50 wt%, at least 1.75 wt%, at least 2.00 wt%, at least 2.25 wt%, at least 2.50 wt%, or at least 2.75 wt% Sb. The formulation may contain less than 3.00 wt%, less than 2.75 wt%, less than 2.50 wt%, less than 2.25 wt%, less than 2.00 wt%, less than 1.75 wt%, less than 1.50 wt%, or less than 1.25 wt% Sb. The formulation may contain at least 1.00 wt% to about 3.00 wt%, at least 1.00 wt% to about 2.50 wt%, about 1.25 wt% to about 2.50 wt%, or about 1.50 wt% to about 2.50 wt% Sb. In some embodiments, the formulation includes about 1.50 wt% to about 2.50 wt% Sb, such as about 1.70 wt%, about 1.80 wt%, about 1.90 wt%, about 2.00 wt%, about 2.10 wt%, or about 2.20 wt%.

[0028] Sb may be included in the formulation, at least in part, in the form of antimony oxide, such as SbO. For example, the formulation may include at least 1.20 wt. % SbO, corresponding to at least 1.00 wt. % Sb. In some embodiments, the formulation includes at least 1.20 wt. % to about 3.60 wt. %, about 1.50 wt. % to about 3.60 wt. %, about 1.80 wt. % to about 3.60 wt. %, about 1.80 wt. % to about 3.40 wt. %, about 1.80 wt. % to about 3.20 wt. %, or about 1.80 wt. % to about 3.00 wt. % SbO. In some embodiments, the formulation includes about 1.80 wt. % to about 3.00 wt. % SbO, such as about 2.00 wt. %, about 2.25 wt. %, about 2.50 wt. %, or about 2.75 wt. % SbO.

[0029] The formulation may comprise at least 0.4 wt% bismuth (Bi), such as at least 0.50 wt%, at least 0.60 wt%, at least 0.70 wt%, at least 0.80 wt%, at least 0.90 wt%, at least 1.00 wt%, at least 1.10 wt%, at least 1.20 wt%, at least 1.30 wt%, at least 1.40 wt%, at least 1.50 wt%, at least 1.60 wt%, at least 1.70 wt%, at least 1.80 wt%, or 1.90 wt%. The formulation may contain less than 2.00 wt%, less than 1.90 wt%, less than 1.80 wt%, less than 1.70 wt%, less than 1.60 wt%, less than 1.50 wt%, less than 1.40 wt%, less than 1.30 wt%, less than 1.20 wt%, less than 1.10 wt%, less than 1.00 wt%, less than 0.90 wt%, less than 0.80 wt%, less than 0.70 wt%, less than 0.60 wt%, or less than 0.50 wt% Bi. The formulation may contain at least 0.40 wt% to about 2.0 wt%, about 0.50 wt% to about 2.00 wt%, about 0.50 wt% to about 1.80 wt%, about 0.50 wt% to about 1.50 wt%, about 0.50 wt% to about 1.20 wt%, or about 0.50 wt% to about 1.00% Bi. In some embodiments, the formulation includes about 0.50 wt% to about 1.00 wt% Bi, such as about 0.60 wt%, about 0.70 wt%, about 0.80 wt%, about 0.90 wt%, or about 1.00 wt% Bi.

[0030] Bi may be included in the formulation, at least in part, in the form of bismuth oxide, such as BiO. For example, the formulation may contain at least 0.44 wt.% BiO, corresponding to at least 0.40 wt.% Sb. In some embodiments, the formulation contains at least 0.44 wt.% to about 2.20 wt.%, about 0.50 wt.% to about 2.20 wt.%, about 0.60 wt.% to about 2.00 wt.%, about 0.60 wt.% to about 1.50 wt.%, or about 0.60 wt.% to about 1.20 wt.% BiO. In some embodiments, the formulation contains about 0.60 wt.% to about 1.20 wt.% BiO, such as about 0.70 wt.%, about 0.80 wt.%, about 0.90 wt.%, about 1.00 wt.%, about 1.10 wt.%, or about 1.20 wt.% BiO.

[0031] The formulation may include Sb and Bi in a molar ratio of Sb:Bi of 2:1 or greater. The molar ratio of Sb to Bi may be, for example, at least 2.5:1, at least 3.0:1, at least 4.0:1, at least 5.0:1, at least 6.0:1, or at least 7.0:1. In some embodiments, the molar ratio of Sb:Bi is from about 2.5:1 to about 5.0:1, such as about 3.0:1, about 4.0:1, about 4.5:1, or about 4.8:1.

[0032] In a first embodiment, the formulation includes, by weight of the formulation, 91.0 wt.% to about 97.0 wt.% ZnO, at least 0.30 wt.% to about 1.00 wt.% Mn, about 1.00 wt.% to about 3.00 wt.% Sb, about 0.40 wt.% to about 2.00 wt.% Bi, and up to 0.50 wt.% Co. For example, such a formulation may include at least 0.39% by weight to about 1.29% MnO (corresponding to at least 0.30% to about 1.00% Mn), about 1.20% to about 3.59% SbO (corresponding to about 1.00% to about 3.00% Sb), about 0.44% to about 2.23% BiO (corresponding to about 0.40% to about 2.00% Bi), and up to about 0.68% COO (corresponding to up to about 0.50% by weight Co).

[0033] In a second embodiment, the formulation includes, by weight of the formulation, 91.0 wt% to about 97.0 wt% ZnO, about 1.50 wt% to about 2.50 wt% Sb, about 0.50 wt% to about 1.00 wt% Bi, about 0.40 wt% to 0.80 wt% Mn, and up to 0.50 wt% Co. For example, the formulation may include about 1.80 wt% to about 2.99 wt% SbO (corresponding to about 1.50 wt% to about 2.50 wt% Sb), about 0.56 wt% to about 1.11 wt% BiO (corresponding to about 0.50 wt% to about 1.00 wt% Bi), about 0.52 wt% to about 1.03 wt% MnO (corresponding to about 0.40 wt% to about 0.80 wt% Mn), and up to about 0.68 wt% CoO (corresponding to up to 0.50% Co).

[0034] In the first and second embodiments, the total amount of Mn and Co can range from about 0.70% to about 1.00% by weight of the formulation. For example, the formulation can include Mn in the form of MnO and Co in the form of Co3O4, with the total amount of MnO and Co3O4 being from about 0.75% to about 1.25% by weight of the formulation.

[0035] Note that in the first and second embodiments, the amount of Co may be reduced to 0.20% or less. For example, the formulation may include 0.27% or less Co3O4 (corresponding to 0.20% or less Co). In the first and second embodiments, the molar ratio of Sb to Bi may be 2:1 or greater, such as 4:1 or greater.

[0036] In a third embodiment, the formulation includes, by weight of the formulation, 91.0 wt% to about 97.0 wt% ZnO, about 0.50 wt% to about 1.00 wt% MnO, about 1.75 wt% to about 2.25 wt% SbO, about 0.50 wt% to 1.00 wt% BiO, and 0.2 wt% or less CoO. In the third embodiment, the total amount of Mn and Co can be about 0.50 wt% to about 1.00 wt% of the formulation, and the molar ratio of Sb to Bi can be 4:1 or greater.

[0037] The formulations described herein may further include one or more additives known in the art, such as Ag, Al, B, Cr, K, Ni, Si, Sn, or combinations thereof. Additives in MOV formulations may facilitate ceramic processing, sintering, and / or microstructural development. Additives may be included in various molecular forms, such as oxides, carbonates, hydroxides, and nitrates. Oxides suitable as additives include, for example, AgO, Ag2O, Al2O3, BO3, Cr2O3, KO, NiO, SiO2, and SnO2. In some embodiments, the formulations may preferably include B and / or K to promote liquid phase formation and may further include other dopants or sintering aids. In some embodiments, the formulations may include Al and / or other dopant species (e.g., 5-20 ppm) to provide electrical conductivity within the ZnO grains.

[0038] Other additives present in the formulation may comprise about 0.001% to 0.500% or more by weight. For example, the formulation may include one or more additives selected from AgO, AgO, AlO, BO, CrO, KO, NiO, SiO, and SnO, each of which, when present, is about 0.005%, about 0.010%, about 0.020%, about 0.030%, about 0.040%, or about 0.045% by weight. In some embodiments, the formulation includes one or more additives selected from AgO, AlO, BO, and KO, each of which, when present, is about 0.020%, about 0.025%, or about 0.030% by weight. In some embodiments, the formulation includes Ag2O, Al2O3, B2O3, and K2O, each of which is present at about 0.020 wt%, about 0.025 wt%, or about 0.030 wt%.

[0039] In some embodiments, each of the first electrode surface and the second electrode surface is at least 4 cm 2 The surface area is approximately 5.0 cm 2 , about 8cm 2 , about 10cm 2 , or about 15 cm 2 In some embodiments, the distance between the first electrode surface and the second electrode surface is at least 15 mm. For example, the distance can be the distance between two metal plates that function as electrodes of a varistor. The distance between the first electrode surface and the second electrode surface can be about 16 mm, about 18 mm, about 20 mm, about 22 mm, or about 25 mm. The surface area and distance can be adjusted depending on the specific application. In some embodiments, each of the first electrode surface and the second electrode surface is at least 4 cm 2 and the distance between the first electrode surface and the second electrode surface is at least 15 mm.

[0040] In some embodiments, the metal oxide varistors described herein have a maximum continuous operating voltage (MCOV) of at least 3.0 kV RMS, such as at least 4.0 kV, at least 4.5 kV, at least 5.0 kV, at least 5.5 kV, at least 6.0 kV, or at least 6.5 kV RMS. For example, the MCOV can be about 4.8 kV, about 5.0 kV, about 5.2 kV, about 5.4 kV, about 5.6 kV, about 5.8 kV, or about 6.0 kV.

[0041] In some embodiments, the metal oxide varistors described herein have a 60 Hz power loss of less than 0.1 W / kV at a maximum continuous operating voltage (MCOV) at 100° C., or less than 0.2 W / kV at a maximum continuous operating voltage (MCOV) at 150° C. The 60 Hz power loss may be less than 0.07 W / kV, less than 0.05 W / kV, or less than 0.02 W / kV at a MCOV at 100° C. The 60 Hz power loss may be less than 0.18 W / kV, less than 0.15 W / kV, less than 0.12 W / kV, or less than 0.10 W / kV at a MCOV at 150° C.

[0042] In some embodiments, the ratio of the power loss at 150° C. to the power loss at 20° C., both measured at 60 Hz in an MCOV, is less than 20. For example, the ratio of the power loss at 150° C. to the power loss at 20° C. can be less than 15, less than 10, less than 5, or less than 2.

[0043] The MOV formulations described herein can be manufactured by methods known in the art. A typical manufacturing process can include pre-reacting the raw materials by mixing or grinding, calcining, recombining with zinc oxide, and pulverizing by spray drying. The powder can then be pressed into the desired shape and heat-treated in one or more steps at temperatures up to 1150°C, during which the organic binder is removed and the material sinters to form a dense ceramic. Typical post-treatments include polishing or metallizing the electrode surface, applying an electrically insulating glaze, and heat-treating.

[0044] MOVs fabricated with the formulations as described herein have performance advantages over existing commercially available MOV devices. In particular, the formulations and MOVs of the present invention exhibit a 1.0-1.25% Tc at high temperatures (e.g., 100-180°C). * It shows reduced power loss in the range of MOV. Due to the reduced risk of thermal runaway, surge arresters may use fewer MOVs to meet functionally equivalent design specifications. When Co is replaced with Mn in the MOV formulation, V=1.2-1.25 * MCOV and power loss at T≧100°C are improved compared to similar formulations containing both Mn and Co. This improvement increases as Mn / (Mn+Co) increases from 0.5 to 1.0. Increasing Mn / (Mn+Co) reduces the nonlinear coefficient (α) at ​​higher current densities, allowing the formulation to be tailored for specific application conditions. The formulation can also be tailored to optimize other properties of interest, such as stability of electrical properties over time under AC excitation and stability under impulse loads.

[0045] As a result of the improved thermal recovery, the MOVs described herein meet or exceed the performance of conventional MOVs typically used in surge arresters. In arrester designs limited by thermal performance, the present disclosure may enable a reduction in the volume of the MOV by 10% or more. This reduction in volume can be achieved, for example, by reducing the cross-sectional area of ​​the MOV block or by reducing the voltage gradient V. res This can be achieved by increasing the V res represents the residual voltage of the MOV for a given impulse wave.

[0046] MOV V res The impulse conditions for measuring V are defined herein as a nominal 8 x 20 μs waveform at 5 kA peak for MOVs used in IEEE "normal load" or IEC "mid-distribution" arresters, and 8 x 20 μs at 10 kA peak for MOVs used in IEEE "heavy load" or IEC "high-distribution" arresters and station-class arresters. res / mm is preferably 420 to 625, most preferably 450 to 525. res / mm is preferably 325 to 525. The voltage gradient can be adjusted by, for example, adjusting the sintering temperature or the MOV composition.

[0047] In addition to improved thermal stability and reduced nonlinearity, the MOVs described herein can be manufactured at significantly lower cost than existing commercially available MOVs by reducing the use of expensive raw materials. Reducing or eliminating cobalt from MOV formulations is strategically important, especially due to price fluctuations and high demand pressures in other commercial applications, such as lithium battery manufacturing. The level of cobalt used can be tailored to the specific needs of a surge arrester design. Completely cobalt-free formulations can meet or exceed the typical performance of known high-voltage MOV devices.

[0048] In another aspect, the present disclosure provides a surge arrester for protecting one or more conductors against power surges, the surge arrester including a metal oxide varistor as described herein, wherein surge current travels from the conductor through the metal oxide varistor to ground. The surge arrester may be constructed using one or more metal oxide varistors in accordance with known methods.

[0049] In some embodiments, the surge arresters described herein are designed for high voltage applications covered by applicable national or international standards, such as IEEE C62.11, IEC 60099-4, or other national standards derived therefrom, and are rated for 4×10 μs surge currents of about 65 kA or greater. [Example]

[0050] Example 1 Exemplary formulations (A, B, and C) were prepared with varying amounts of Mn and Co according to Table 1. The properties of these formulations are shown in Table 2. The "REF" material is a commercially available MOV used in IEEE C62.12 "normal duty" surge arresters. The nonlinearity coefficient (α) values ​​are calculated from 3 mA to 5 kA for all MOVs with similar physical dimensions. Each α value represents the average value from 3 mA to 5 kA. All three formulations exhibit α values ​​equal to or greater than that of the commercially available material. It is noteworthy that the α value decreases when Co is replaced with Mn. [Table 1] [Table 2]

[0051] The power loss (expressed as RMS watts per kV of MCOV) in the MCOV was measured for three example formulations and a commercially available MOV ("REF"), as shown in Figure 1. Formulations A, B, and C demonstrated approximately an order of magnitude improvement in power loss in the MCOV compared to the commercial MOV. These formulations also demonstrated a reduced temperature dependence of power loss. Formulation A (0% Co3O4) and Formulation B (0.09% Co3O4) further reduced power loss compared to Formulation C (0.54% Co3O4) in the temperature range of approximately 75 to 180°C, which is attributed to the substitution of Mn for Co in these formulations.

[0052] Additional MOV formulations were prepared and tested. Table 3 shows the weight percent of the elements contained in the formulations, calculated from the corresponding oxides. Formulations B, D, and G performed best overall under the test conditions. [Table 3]

[0053] Various features and advantages of the invention are set forth in the following claims.

Claims

1. 1. A metal oxide varistor comprising: about 91.0 wt. % to about 97.0 wt. % ZnO; at least 0.3 wt. % to about 1.0 wt. % Mn; at least 0.4 wt. % to about 2.0 wt. % Bi; at least 1.0 wt. % to about 3.0 wt. % Sb; and Sintered ceramics containing 0 to 0.3 wt. % Co (wherein manganese fluoride is MnF ２ and nickel fluoride is converted to NiF ２ (excluding those containing 0.05 mol % or more in the form of MnO and Co in the sintered ceramic ３ O ４ a total amount of from about 0.75% to about 1.25% by weight.

2. 2. The metal oxide varistor of claim 1, wherein the sintered ceramic comprises 0.2 wt. % or less Co based on the weight of the sintered ceramic.

3. 2. The metal oxide varistor of claim 1, wherein the sintered ceramic comprises 0.1 wt % or less Co based on the weight of the sintered ceramic.

4. 2. The metal oxide varistor of claim 1, wherein the sintered ceramic is Co-free.

5. The sintered ceramic comprises: about 0.4 wt.% to about 0.8 wt.% Mn; about 0.5 wt. % to about 1.0 wt. % Bi; and 10. The metal oxide varistor of claim 1, comprising about 1.5% to about 2.5% by weight of Sb.

6. 6. The metal oxide varistor according to claim 1, wherein the molar ratio of Sb to Bi is 2:1 or more.

7. 7. The metal oxide varistor according to claim 1, wherein the molar ratio of Sb to Bi is 4:1 or more.

8. 8. The metal oxide varistor of claim 1, wherein the sintered ceramic further comprises an additive comprising Ag, Al, B, Cr, K, Ni, Si, Sn, or a combination thereof.

9. and a second electrode having a second electrode surface, each of the first electrode surface and the second electrode surface being 4 cm ２ and the distance between the first electrode surface and the second electrode surface is at least 15 mm.

10. The metal oxide varistor of any one of claims 1 to 9, wherein the metal oxide varistor has a maximum continuous operating voltage (MCOV) of at least 3.0 kV RMS.

11. 11. The metal oxide varistor of claim 10, wherein the maximum continuous operating voltage (MCOV) is at least 4.5 kV RMS.

12. 12. A metal oxide varistor according to any one of claims 1 to 11, wherein the metal oxide varistor has a 60 Hz power loss of less than 0.1 W / kV at a maximum continuous operating voltage (MCOV) at 100°C or less than 0.2 W / kV at a maximum continuous operating voltage (MCOV) at 150°C.

13. 13. A metal oxide varistor according to any one of claims 1 to 12, wherein the ratio of power loss at 150°C to power loss at 20°C is less than 20 when both are measured at 60Hz, MCOV.

14. A surge arrester for protecting one or more conductors against power surges, the surge arrester comprising a metal oxide varistor according to any one of claims 1 to 13, wherein surge current travels from the conductors to ground through the metal oxide varistor.

15. 15. The surge arrester of claim 14, designed for high voltage applications covered by a national or international standard, such as IEEE C62.11 or IEC 60099-4, or a national standard derived therefrom, and rated for a 4 x 10 μs surge current of greater than or equal to about 65 kA.

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