US20090142217A1

US20090142217A1 - Composition and method

Info

Publication number: US20090142217A1
Application number: US12/186,552
Authority: US
Inventors: Daniel Qi Tan
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 2007-12-03
Filing date: 2008-08-06
Publication date: 2009-06-04
Also published as: US20090142580A1; US20090143216A1; US20090140833A1; US8217751B2; US8207813B2

Abstract

A method includes contacting a transition metal oxide, a sintering additive, and a grain growth inhibitor additive to form a mixture. The transition metal oxide include particles that have an average diameter less than about 1 micrometer and treating the mixture to a temperature profile that is sufficiently high that a sintered mass is formed from the mixture. The thermal profile is less than about 1050 degrees Celsius.

Description

RELATED APPLICATIONS

This application is a non-provisional application that claims priority to provisional U.S. Pat. application Ser. No. 60/991,871, filed Dec. 03, 2007; the disclosure of which is hereby incorporated by reference.

BACKGROUND

1. Technical Field
The invention includes embodiments that relate to a composition for use as a surge protector and/or varistor. The invention includes embodiments that relate to a method of making and/or using the composition, or derived device.
2. Discussion of Art
A varistor is an electronic component with a non-ohmic current-voltage characteristic. Varistors may protect circuits against excessive transient voltages by incorporating them into the circuit in such a way that, when triggered, they will shunt the current created by the high voltage away from the sensitive components. A varistor may be known as Voltage Dependent Resistor or VDR.
A type of varistor is the Metal Oxide Varistor (MOV). This contains a ceramic mass of zinc oxide grains, in a matrix of other metal oxides (such as small amounts of bismuth, cobalt, manganese) sandwiched between two metal plates (the electrodes). The boundary between each grain and its neighbour forms a diode junction, which allows current to flow in only one direction. The mass of randomly oriented grains is electrically equivalent to a network of back-to-back diode pairs, each pair in parallel with many other pairs. When a small or moderate voltage is applied across the electrodes, only a tiny current flows, caused by reverse leakage through the diode junctions. When a large voltage is applied, the diode junctions break down because of the avalanche effect, and a large current flows. The result of this behaviour is a highly nonlinear current-voltage characteristic, in which the MOV has a high resistance at low voltages and a low resistance at high voltages.
A varistor remains non-conductive as a shunt mode device during normal operation when voltage remains well below its “clamping voltage”. If a transient pulse (often measured in joules) is too high, the device may melt, burn, vaporize, or otherwise be damaged or destroyed. This unacceptable (catastrophic) failure occurs when “Absolute Maximum Ratings” are exceeded. Varistor degradation is defined using curves that relate current, time, and number of transient pulses. A varistor fully degrades when its “clamping voltage” has changed by 10 percent. A fully-degraded varistor may remain functional, having no catastrophic failure, and may not be visually damaged.
It may be desirable to have a method that differs from those methods currently available to provide a composition or article with properties and characteristics that differ from those properties of currently available compositions and articles.

BRIEF DESCRIPTION

In one embodiment, a method is provided that includes contacting a transition metal oxide, a sintering additive, and a grain growth inhibitor additive to form a mixture, and treating the mixture to a temperature profile that is sufficiently high that a sintered mass is formed from the mixture, and the thermal profile is less than about 1050 degrees Celsius. The transition metal oxide comprises particles that have an average diameter less than about 1 micrometer.
In one embodiment, a method provides a sintered mass having a plurality of cores and a grain boundary layer disposed between each of the plurality of cores. The core includes a transition metal oxide, and grain boundary layer includes a sintering additive and a grain growth inhibitor additive.
In one embodiment, a method includes contacting a transition metal oxide, and a sintering additive and calcining the transition metal oxide and the sintering additive together, wherein the calcining comprises heating to a temperature of about 400 degress Celsius to provide a calcined mass. The calcined mass is contacted with a grain growth inhibitor additive to form a mixture and treating the mixture to a temperature profile that is sufficiently high that a sintered mass is formed from the mixture, and the thermal profile is less than about 1050 degrees Celsius is provided. The transition metal oxide comprises particles that have an average diameter less than about 1 micrometer.
In one embodiment, a method includes contacting a transition metal oxide, a sintering additive, and a grain growth inhibitor additive to form a mixture, and treating the mixture to a temperature profile that is sufficiently high that a sintered mass is formed from the mixture, and the thermal profile is less than about 1050 degrees Celsius. The sintered mass can be contacted with an electrically conductive metal to form a electrical connection therebetween.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a block diagram of a method to make a sintered mass in accordance with one embodiment of the invention.

FIG. 2 shows a block diagram of a method to make a sintered mass in accordance with one embodiment of the invention.

FIG. 3 shows a graph of the electric field versus the current density (current voltage graph) for a composition in accordance with one embodiment of the invention and a comparative sample.

FIG. 4 shows SEM micrographs of a composition in accordance with an embodiment and a control blank.

FIG. 5 shows SEM micrographs of a composition in accordance with one embodiment of the invention.

DETAILED DESCRIPTION

The invention includes embodiments that relate to a composition for use as a surge protector and/or varistor. The invention includes embodiments that relate to a method of making and/or using the composition, or the derived device.
As used herein, the term sintering is a method for making objects from particles or powder by heating the material (below its melting point) until its particles adhere to each other. Sintered refers to particles or powder that has undergone a sintering process. A sintered mass refers to the formed shape that is the result of the sintering of powders or particulate. In the sintered mass, formerly discrete particles or powder grains retain a core, and the interstitial area from one core to another core is at least partially filled with a grain boundary layer that separates the cores.
In one embodiment, a composition includes a sintered mass. The sintered mass includes a plurality of particle cores and a grain boundary layer disposed between each of the plurality of particle cores. Each of the cores may include a transition metal oxide. The grain boundary layer includes a sintering additive, a grain boundary additive, and/or a breakdown voltage additive.
In one embodiment, the particle core may include a transition metal. In one embodiment, the transition metal may be a transition metal oxide. Examples of transition metal oxides include but are not limited to zinc oxide, tin oxide, and titanium oxide. In one embodiment, the transition metal oxide includes a zinc oxide. The amount of the transition metal oxide, by weight, may be greater than about 80 percent based on the total weight of the sintered mass. In one embodiment, the amount may be in a range of from about 80 weight percent to about 85 weight percent, from about 85 weight percent to about 90 weight percent, or from about 90 weight percent to about 95 weight percent, or from about 95 weight percent to about 98 weight percent based on the total weight of the sintered mass.
In one embodiment, the grain boundary layer is disposed between each of the plurality of the cores. The grain boundary layer includes a sintering additive. In one embodiment, the sintering additive may include one or more of aluminum, lithium, antimony, bismuth, cobalt, chromium, manganese, nickel, magnesium, or silicon. The sintering additive may include a combination of two or more of the foregoing. In one embodiment, the sintering additive includes one or more of SiO₂, Mn₂O₃, NiO, MnO₂, or MnCO₃. In one embodiment, the sintering additive may include one or more of Li₂CO₃, or LiBiO₃. In one embodiment, the sintering additive may include only one of the foregoing. The selection of the sintering additive may be based on one or more factors as the sintering additives differ in efficacy and effect. Such factors may include the desired sintering temperature, the sintering pressure, the material performance, and the desired grain characteristics.
The sintering additive may be present in an amount that is less than about 15 percent by weight, based on the total weight of the sintered mass. In one embodiment, the sintering additive amount may be in a range of from about 15 percent to about 12 percent, from about 12 percent to about 10 percent, from about 10 percent to about 8 percent, from about 8 percent to about 4 percent, from about 4 percent to about 2 percent, from about 2 percent to about 0.5 percent, from about 0.5 percent to about 0.3 percent, or from about 0.3 percent to about 0.1 percent, or from about 0.1 percent to about 0.03 percent.
In one embodiment, the grain boundary includes a grain growth inhibitor additive. In one embodiment, the grain growth inhibitor additive may include one or more of Sb₂O₃, CaO, Al₂O₃, MgO, or Fe₂O₃. In one embodiment, the grain growth inhibitor may consist essentially of only one of the foregoing. The selection of the grain growth inhibitor additive may be based on one or more factors as the grain growth inhibitor additive differ in efficacy and effect. Such factors may include the desired sintering temperature, the sintering pressure, the material performance, and the desired grain characteristics. In one embodiment, the grain growth inhibitor additive may inhibit grain growth to maintain relatively smaller grains. The grain growth inhibitor additive may control the grain size distribution, as well. In one embodiment, the grain growth inhibitor additive may be present in an amount in a range of from about 0.1 weight percent to about 0.5 weight percent, from about 0.5 weight percent to about 1.5 weight percent, or from about 1.5 weight percent to about 3 weight percent.
In one embodiment, the grain growth inhibitor additive may include a combination of two or more of the foregoing. In one embodiment, the grain growth inhibitor additive may be present in the sintered mass in an amount, by weight, that is less than about 10 percent based on the total weight of the sintered mass. In one embodiment, the grain growth inhibitor additive amount may be in a range of from about 10 weight percent to about 8 weight percent, from about 8 weight percent to about 6 weight percent, 6 weight percent to about 4 weight percent, from about 4 weight percent to about 2 weight percent, from about 2 weight percent to about 1 weight percent, from about 1 weight percent to about 0.5 weight percent, from about 0.5 weight percent to about 0.1 weight percent, or less than about 0.1 weight percent.
In one embodiment, the composition may further include a grain boundary additive. In one embodiment, the grain boundary additive includes a breakdown voltage additive. In one embodiment, the grain boundary additive may enhance the grain boundary barrier. In one embodiment, the grain boundary additive may include one or more of Co₃O₄, Co₂O₃, Cr₂O₃, Bi₂O₃, Pr₂O₃, NiO, or SnO₂. In one embodiment, the grain boundary additive consists essentially of only one of the foregoing. The selection of the grain boundary additive may be based on one or more factors as the grain boundary additive differ in efficacy and effect. Such factors may include the desired sintering temperature, the sintering pressure, the material performance, and the desired grain characteristics. The grain boundary additive may be present in an amount less than about 1 weight percent. In one embodiment, the grain boundary additive may be present in an amount in a range of from about 0.01 weight percent to about 0.5 weight percent, from about 0.5 weight percent to about 0.75 weight percent, or from about 0.75 weight percent to about 1 weight percent. In one embodiment, the composition is free of Co₂O₃. In another embodiment, the amount of Co₂O₃is less than about 0.05 weight percent.
In one embodiment, the additive may include a combination of two or more of the foregoing. In one embodiment, the grain boundary additive may be present in the sintered mass in an amount, by weight, that is less than about 10 percent based on the total weight of the sintered mass. In one embodiment, the grain boundary additive is present in an amount in a range of from about 10 weight percent to about 8 weight percent, from about 8 weight percent to about 6 weight percent, from about 6 weight percent to about 4 weight percent, from about 4 weight percent to about 2 weight percent, from about 2 weight percent to about 1 weight percent, from about 1 weight percent to about 0.5 weight percent, from about 0.5 weight percent to about 0.1 weight percent, or less than about 0.1 weight percent.
In one embodiment, the average distance from one core to an adjacent core in the plurality of cores is less than about 1 micrometer. In one embodiment, the average distance may be in a range of from about 1 micrometer to about 0.8 micrometers, or from about 0.8 micrometers to about 0.5 micrometers. In another embodiment, the average distance may be in a range of from about 500 nanometers to about 400 nanometers, from about 400 nanometers to about 300 nanometers, from about 300 nanometers to about 250 nanometers, from about 250 nanometers to about 200 nanometers, from about 200 nanometers to about 150 nanometers, from about 150 nanometers to about 100 nanometers, from about 100 nanometers to about 50 nanometers, or less than about 50 nanometers.
In one embodiment, the average diameter of the core in the plurality of cores is less than about 1 micrometer. In one embodiment, the average diameter may be in a range of from about 1 micrometer to about 0.8 micrometers, or from about 0.8 micrometers to about 0.5 micrometers. In another embodiment, the average diameter may be in a range of from about 500 nanometers to about 400 nanometers, from about 400 nanometers to about 300 nanometers, from about 300 nanometers to about 250 nanometers, from about 250 nanometers to about 200 nanometers, from about 200 nanometers to about 150 nanometers, from about 150 nanometers to about 100 nanometers, from about 100 nanometers to about 50 nanometers, or less than about 50 nanometers.
The micro-structure or nano-structure of the composition may be expressed in terms of an average distance from one core to an adjacent core in the sintered mass. The average distance from one core to an adjacent core in the sintered mass may be less than 5 micrometers. In one embodiment, the average distance may be in a range of from about 1 micrometer to about 0.8 micrometers, or from about 0.8 micrometers to about 0.5 micrometers. In another embodiment, the average distance may be in a range of from about 500 nanometers to about 400 nanometers, from about 400 nanometers to about 300 nanometers, from about 300 nanometers to about 250 nanometers, from about 250 nanometers to about 200 nanometers, from about 200 nanometers to about 150 nanometers, from about 150 nanometers to about 100 nanometers, from about 100 nanometers to about 50 nanometers, or less than about 50 nanometers. An exemplary core-to-core average distance may be in a range of from about 35 nanometers to about 75 nanometers.
The distance of one core to another core, coupled with the core size, may affect the average thickness of the grain boundary layer. In one embodiment, the average thickness of the grain boundary layer may be less than about 1 micrometer. In another embodiment, the average thickness may be in a range of from about 1 micrometer to about 0.8 micrometers, or from about 0.8 micrometers to about 0.5 micrometers. In yet another embodiment, the average thickness may be in a range of from about 500 nanometers to about 400 nanometers, from about 400 nanometers to about 300 nanometers, from about 300 nanometers to about 250 nanometers, from about 250 nanometers to about 100 nanometers, from about 100 nanometers to about 50 nanometers, from about 50 nanometers to about 35 nanometers, from about 35 nanometers to about 20 nanometers, or less than about 20 nanometers.
The grain boundary layer thickness, may be expressed as a mean value in nanometers. The mean value for the grain boundary layer may be less than about 50 nanometers. In one embodiment, the mean value may be in a range of from about 50 nanometers to about 10 nanometers, from about 10 nanometers to about 1 nanometer, or from about 1 nanometer to about 0.1 nanometers.
In addition to such factors as the uniformity of core diameters, the uniformity of distribution of materials, and the uniformity of the grain boundary layer, the average distance of the cores from one to another may affect the performance, properties and characteristics of the varistor device made therefrom. Particularly, the diode junction performance, and the number of diode junctions per unit volume, may flow directly from the core spacing parameter.
In one embodiment, the sintered mass may have a dielectric strength or breakdown field of greater than about 0.5 kV/mm. In one embodiment, the dielectric strength or breakdown field is in a range of from about 0.5 kV/mm to about 1 kV/mm, from about 1 kV/mm to about 1.5 kV/mm, from about 1.5 kV/mm to about 2 kV/mm, from about 2 kV/mm to about 2.5 kV/mm, from about 2.5 kV/mm to about 2.8 kV/mm, or greater than about 2.8 kV/mm. In one embodiment, the sintered mass may have a non-linearity coefficient (α) of greater than 25. In one embodiment, the non-linearity coefficient (α) may be in a range of from about 25 to about 50, from about 50 to about 75, from about 75 to about 100, from about 100 to about 125, from about 125 to about 140, or greater than about 140.
The thermal profile may play a role in the melt temperature of the electrode of the MOV device. If the thermal profile is higher than the electrode melt temperature, then the electrode may be melted, damaged or destroyed. A higher thermal excursion during manufacture or sinter may then require an electrode with a corresponding melt temperature suitable for use after exposure to that temperature. Lower temperature capable electrode materials may be economically desirable, if the other performance parameters are correct. In addition, if the thermal profile shows a temperature excursion too high, the micro-structure or nano-structure may change and the sintered particles may melt and flow together rather than remain as a sintered mass. This may need to be balanced, as at least some heat is needed to get the particles to sinter in the first instance.
In one embodiment, a sintered mass may be produced by mixing a transition metal oxide, a sintering additive, and a grain boundary additive under defined conditions to form a mixture. The mixture can be treated to a determined temperature profile. In one embodiment, the temperature profile includes exposure to a sinter temperature of less than about 1050 degrees Celsius. The composition may have a thermal profile also known as thermal history that may include exposure to a sintering temperature of not greater than about 1050 degrees Celsius. In one embodiment, the thermal profile includes exposure to a sinter temperature in a range of from about 1050 degrees Celsius to about 1000 degrees Celsius, from about 1000 degrees Celsius to about 950 degrees Celsius, from about 950 degrees Celsius to about 900 degrees Celsius, from about 900 degrees Celsius to about 875 degrees Celsius, or from about 875 degrees Celsius to about 850 degrees Celsius.
In one embodiment, the method includes calcining the transition metal oxide and the sintering additive together before forming the mixture. Calcining of the transition metal oxide and the sintering additive includes heating to a temperature that is greater than about 400 degrees Celsius. In one embodiment, calcining includes exposure of the mixture to a temperature in a range of from about 400 degrees Celsius to about 450 degrees Celsius, from about 450 degrees Celsius to about 550 degrees Celsius, from about 550 degrees Celsius to about 600 degrees Celsius, from about 600 degrees Celsius to about 650 degrees Celsius, or from about 650 degrees Celsius to about 800 degrees Celsius.
In one embodiment, a method includes contacting a transition metal oxide with a sintering additive to form a premix, wherein the transition metal oxide comprises particles that have an average diameter less than about 1 micrometer. The premix may be calcined. The clacining includes heating to a temperature of about 400 degress Celsius to provide a calcined mass. The calcined mass may be contacted with a grain growth inhibitor additive to form a mixture. The mixture is sintered at a temperature profile that is sufficiently high that a sintered mass is formed from the mixture, and the thermal profile is less than about 1050 degrees Celsius.
In one embodiment, the method includes contacting the sintered mass with an electrically conductive material to form an electrical connection therebetween. The electrically conductive material electrode may be formed by pressing, heating, melt flowing, casting, printing, metallizing/etching, and the like. Mechanical methods of attachment may be available in some embodiments. Alternatively, a precursor material may be disposed on the sintered mass and converted into an electrically conductive material. In one embodiment, the electrically conductive materials may include one or more of platinum, palladium, copper, silver, tin, aluminum, iron, carbon, nickel, antimony, chromium, or gold. Alloys of the foregoing are also suitable based on application specific parameters (such as brass or Ni—Sn). In one embodiment, the electrically conductive material consists essentially of silver. In one embodiment, the electrically conductive material includes carbon, and the carbon is amorphous or structured (such as in a nanotube or nanowire).
A block diagram 10 of one embodiment of a method is provided in FIG. 1. The transition metal oxide and the sintering additive and the grain growth inhibitor additive are provided or obtained 12. A solvent wets the transition metal oxide and the sintering additive and the grain growth inhibitor additive to form a wet mixture 14. The wet mixture is dried to provide a dry mixture 16. The dry mixture is powdered to provide a powdered dry mixture 18. The powdered dry mixture is calcined to form a calcined mass 20. The calcined mass is compacted to form a powdered mass 22. The powdered mass is sintered at a temperature that is less than about 1050 degrees Celsius to provide the sintered mass 24.
A block diagram 30 of one embodiment of a method is provided in FIG. 2. A transition metal oxide is provided 32. A solvent wets the transition metal oxide to form a wet slurry 34. The wet slurry is dried to provide a dry mixture 36. The dry mixture is powdered to provide a powdered dry mixture 38. The powdered dry mixture is calcined to form a calcined mass 40. The calcined mass is contacted with the sintering additive and the grain growth inhibitor additive 42 to form a calcined mixture. A solvent wets the calcined mixture to form a wet calcined mixture 44. The wet calcined mixture is dried to provide a dry calcined mixture 46. The dry mass is powdered to provide a powdered mass 48. The powdered mass is calcined to provide a calcined powdered mass 50. The calcined powdered mass is sintered at a temperature that is less than 1050 degrees Celsius to provide the sintered mass 52.
In one embodiment, the composition includes a sintered reaction product of transition metal oxide particles that have an average diameter that is less than about 1 micrometer; and sintering additive particles having an average diameter that is less than about 1 micrometer. The grain growth inhibitor additive particles may have an average diameter that is less than about 1 micrometer. Due to the change in available surface area, and packing tendencies, particles of different sizes may form sintered masses having differing properties and characteristics.
In one embodiment, the composition includes a sintered mass of particles that may include a transition metal oxide, a sintering additive, and a grain growth inhibitor additive. The sintered mass may have a density that is greater than 98 percent of theoretical density for a composition comprising the transition metal oxide.
In one embodiment, the composition includes sintered particles that include a transition metal oxide, a sintering additive, and a grain growth inhibitor additive and defining grains. The grains may have grain boundaries that define the grains to have an average grain size of less than about 0.8 micrometers. In one embodiment, the method includes contacting a transition metal oxide, a sintering additive, and a grain growth inhibitor additive to form a mixture. The mixture may be treated to a temperature profile. In one embodiment, the temperature profile includes exposure to a sinter temperature of less than about 1050 degrees Celsius.

EXAMPLES

The following examples illustrate methods and embodiments in accordance with the invention, and as such do not limit the claims. Unless specified otherwise, all ingredients may be commercially available from such common chemical suppliers as Alpha Aesar, Inc. (Ward Hill, Mass.), Sigma Aldrich (St. Louis, Mo.), Spectrum Chemical Mfg. Corp. (Gardena, Calif.), and the like.
Examples 1 through 3 are prepared by mixing, calcining, ball milling, and sintering. The sintering is performed in a Uniaxial Press to make a puck for each sample that is about 1 inch in diameter. The various components and the weight percent for each of the components for examples 1 to 3 are given in Table 1.

TABLE 1

Composition
(Weight		Sample	Sample	Comparative	Comparative
percent)	Sample 1	2	3	Sample 1	Sample 2

ZnO	94	85.5	94.69	83.39	92.21
Bi₂O₃	0.5	2	3	2.12	1.40
Sb₂O₃	1	3	1.5	6.34	3.75
Al₂O₃	—	2	0.01	0.04	—
SiO₂	2	3	—	0.43	0.07
Cr₂O₃	0.5	—	—	—	1.02
MnO	—	—	—	0.9	0.4
Mn₂O₃	0.5	—	0.1	—	—
MgO	—	2	—	—	—
Fe₂O₃	—	—	—	0.01	0.04
Co₂O₃	—	—	—	1.13	1.17
Co₃O₄	0.5	2.5	0.5	—	—
NiO	1	—	0.2	1.27	—
SnO₂	—	—	—	—	0.93

Example 1

A mixture is formed from zinc oxide, and additives selected from cobalt, antimony, nickel, and chromium oxide nanopowders with bismuth, silicon, manganese oxide nanopowders in a ratio given in Table 1. The zinc oxide is commercially obtainable from Horsehead Corporation, (Monaca, Pa.). The additives are commercially obtainable from Nanostructured and Amorphous Materials Inc. (Houston, Tex.).
The materials form a mixture in a mixed oxide wet process. The mixture is milled in a ball mill for about 6 hours in a ratio materials:ball:isopropyl alcohol=1:5:2 to form a slurry. The slurry is dried at 100 degrees Celsius. The dried powder is sieved and calcined at 550 degrees Celsius for about 2 hours in a Thermolyne 1400 furnace. The calcined powder is then ball milled for about 4 hours. The slurry formed is dried at 100 degrees Celsius and the dried powder is sieved. The powder is then pressed into pellets (thickness of about 1.5 millimeters) with a force of about 10000 pounds for about 1 minute. The pellet is sintered at temperatures from about 1000 degrees Celsius and 1050 degrees Celsius. The sintering is done in two different profiles including one and two steps in a Uniaxial Press for about 2 hours. The first profile is carried out at about at 1050 degrees Celsius at a heating rate of about 5 degrees Celsius per minute for about 2 hours and is allowed to cool. The second profile is carried out at about at 1000 degrees Celsius at a rate of about 10 degrees Celsius per minute for about 0.1 hours. Following this a second step sintering at a temperature of about 925 degrees Celsius to 975 degrees Celsius at a heating rate of about 10 degrees Celsius per minute is carried out for about 2 hours. The resultant product is Sample 1, which has the compositional distribution as indicated in Table 1.

Example 2

A mixture is formed from zinc oxide, and additives selected from oxide nanopowders cobalt, and antimony, with nanopowder oxides of bismuth, silicon, aluminum and magnesium in a ratio given in Table 1.
The materials are mixed using a mixed Oxide Wet Process. The mixture is milled in a ball mill for about 6 hours in a ratio materials:ball:isopropyl alcohol=1:5:2 to form a slurry. The slurry is dried at 100 degrees Celsius. The dried powder is sieved and calcined at 550 degrees Celsius for about 2 hours in a Thermolyne 1400 furnace. The calcined powder is then ball milled for about 4 hours. The slurry formed is dried at 100 degrees Celsius and the dried powder is sieved. The powder is then pressed into pellets (thickness of about 1.5 millimeters) with a force of about 10000 pounds for about 1 minute. The pellet is sintered in a Uniaxial Press at different temperatures for about 2 hours at about 950 degrees Celsius, about 1050 degrees Celsius at a rate of about 5 degrees Celsius per minute. The resultant product is Sample 2, which has the compositional distribution indicated in Table 1.

Example 3

A mixture is formed from zinc oxide (from Horsehead Corporation, Monaca, Pa.), and additives selected from powders of cobalt, nickel, and antimony-based materials (from Nanostructured and Amorphous Materials Inc, Houston, Tex.), and with powders of bismuth, aluminum and manganese-based materials in amounts as given in Table 1.
The materials are mixed using a mixed oxide wet process. The mixture is milled in a ball mill for about 6 hours in a ratio of powder materials:ball:isopropyl alcohol=1:5:2 to form a slurry. The slurry is dried at 100 degrees Celsius. The dried powder is sieved and calcined at 550 degrees Celsius for about 2 hours in a Thermolyne 1400 furnace. The calcined powder is then ball milled for about 4 hours. The slurry formed is dried at 100 degrees Celsius and the dried powder is sieved. The powder is then pressed into pellets (thickness of about 1.5 millimeters) with a force of about 10000 pounds for about 1 minute. The pellet is sintered in a pressureless mode at different temperatures for about 2 hours at about 850 degrees Celsius, about 900 degrees Celsius, about 950 degrees Celsius, about 1000 degrees Celsius and about 1050 degrees Celsius at a rate of about 5 degrees Celsius per minute. The resultant product is Sample 3, which has the compositional distribution indicated in Table 1.

Current—Voltage (I-V) Measurement:

A 10 kiloOhm or 100 MegaOhm resistor is connected in parallel to the varistor and a voltage is applied. V1, the total voltage on sample and varistor is measured using a high voltage probe. V2, the voltage on the resistor is measured by a multimeter. V2 is used to calculate the current flowing through the varistor. V1-V2 is the voltage on the Samples 1-6. To measure I-V curve, at low voltage, a 100 MOhm resistor is used until the voltage on it is higher than about 100 Volts. A 10 kilo Ohm resistor is used to measure the I-V curve under high voltage (higher than about 100 Volts).
FIG. 3 shows the results of metal oxide varistor materials of Samples 1-3 relative to a commercially available metal oxide varistor material. The materials of Samples 1-3 display relatively better breakdown strength and relatively better nonlinearity compared to Comparative Sample 1. The breakdown fields (electric fields when current density is 1 milliAmp per square centimeter) and nonlinearity coefficient α calculated are summarized in Table 2.

TABLE 2

Breakdown fields and nonlinearity of metal oxide varistor materials of
Samples 1-3 and commercially available metal oxide varistor material

	Sintering
	Temperature	Breakdown
	(° C.)/	Field	Nonlinearity
Composition	Time (Hours)	(V/mm)	coefficient α

Sample

1	1000/2	1343	63
		1050/2	972	138
	Sample 2	950/2	2800	40
		1000/2	2216	18
	Sample 3	850/2	1710	77
		900/2	546	19
		950/2	515	77
		1000/2	400	42
		1050/2	315	79
	Comparative	NA	125	22
	Sample 1

Table 2 shows that the metal oxide varistor materials of Samples 1-3 perform better after low temperature firing. For example, Sample 3 gives a breakdown field of greater than about 1700 volts per millimeter and a good nonlinearity coefficient (α) of about 77, but still has a low sintering temperature of 850 degrees Celsius.
Microstructure formation may depend at least in part on the sintering profile. Grain size is found to increase at higher sintering temperature. FIGS. 4-5 compare the microstructure of a commercially available metal oxide varistor material Comparative Sample 1 (FIG. 4) with a metal oxide varistor material of Example 3 (FIG. 5). The average grain size of the metal oxide varistor materials of Sample 3 (sintered at 850 degrees Celsius) is less than 1 micrometer; and, this is in comparison to the commercially available metal oxide varistor material Comparative Sample 1 that has a grain size that is greater than 10 micrometers. Several phases may coexist in the metal oxide varistor materials of Samples 1-3. These phases may include the major conductive phase of less than 1 micrometer in size and one or more secondary phases located at the grain boundaries and in the grain boundary layer, which itself may include various dopants and sintering additives.

Example 4

A mixture is formed from zinc oxide, and additives selected from oxide powders cobalt, and antimony, with powder oxides of bismuth, silicon, aluminum and chromium in amounts as given in Table 3. Unless otherwise indicated, the powders are nanoscale and have a narrow size distribution.
The materials are mixed using a mixed Oxide Wet Process. The mixture is milled in a ball mill for about 6 hours in a ratio of ingredients:ball:isopropyl alcohol of 1:5:2 to form a slurry. The slurry is dried at 100 degrees Celsius. The dried powder is sieved and calcined at 550 degrees Celsius for about 2 hours in a THERMOLYNE 1400 furnace. The calcined powder is then ball milled for about 4 hours. The slurry formed is dried at 100 degrees Celsius and the dried powder is sieved. The powder is then pressed into pellets (thickness of about 1.5 millimeters) with a force of about 10000 pounds for about 1 minute. The pellet is sintered in a Uniaxial Press at different temperatures for about 2 hours at about 950 degrees Celsius, about 1050 degrees Celsius at a rate of about 5 degrees Celsius per minute. The resultant product is Sample 4, which has the compositional distribution indicated in Table 3.

Example 5

A plurality of mixtures are formed from zinc oxide, and additives selected from cobalt, antimony, nickel, and chromium-based powders with bismuth, silicon, manganese-based powders, each in an amount as given in Table 3. The powders, unless context or language indicates otherwise, are nano-scale and have an average diameter that is less than 100 nanometers, and a relatively narrow and mono-modal size distribution.
The materials form a mixture in a mixed oxide wet process. The mixture is milled in a ball mill for about 6 hours in a ratio ingredients:ball:isopropyl alcohol of 1:5:2 to form a slurry. The slurry is dried at 100 degrees Celsius. The dried powder is sieved and calcined at 550 degrees Celsius for about 2 hours in a Thermolyne 1400 furnace. The calcined powder is then ball milled for about 4 hours. The slurry formed is dried at 100 degrees Celsius and the dried powder is sieved. The powder is then pressed into pellets (thickness of about 1.5 millimeters) with a force of about 10,000 pounds for about 1 minute. The pellet is sintered at temperatures from about 1000 degrees Celsius and 1050 degrees Celsius. The sintering is done in two different profiles including one and two steps in a Uniaxial Press for about 2 hours. The first profile is carried out at about at 1050 degrees Celsius at a heating rate of about 5 degrees Celsius per minute for about 2 hours and is allowed to cool. The second profile is carried out at about at 1000 degrees Celsius at a rate of about 10 degrees Celsius per minute for about 0.1 hours. Following, a second step includes sintering at a temperature of about 925 degrees Celsius to 975 degrees Celsius. The sintering is at a heating ramp up rate of about 10 degrees Celsius per minute for about 2 hours. The resultant product is Sample 5, which has the compositional distribution as indicated in Table 3.

Example 6

A mixture is formed from zinc oxide, and additives selected from powders of cobalt, lithium, nickel, and antimony-based materials, with powders of bismuth, and aluminum-based materials in amounts as given in Table 3.
The materials are mixed using a mixed oxide wet process. The mixture is milled in a ball mill for about 6 hours in a ratio of powder ingredients:ball:isopropyl alcohol of 1:5:2 to form a slurry. The slurry is dried at 100 degrees Celsius. The dried powder is sieved and calcined at 550 degrees Celsius for about 2 hours in a THERMOLYNE 1400 furnace. The calcined powder is then ball milled for about 4 hours. The slurry formed is dried at 100 degrees Celsius and the dried powder is sieved. The powder is then pressed into a plurality of pellets (thickness of about 1.5 millimeters) with a force of about 10000 pounds for about 1 minute.
The pellets are sintered in a pressureless mode at different temperatures for about 2 hours. The temperatures are: about 800 degrees Celsius (sample 6), about 850 degrees Celsius (sample 7), about 900 degrees Celsius (sample 8), about 950 degrees Celsius (sample 9), about 1000 degrees Celsius (sample 10), and about 1050 degrees Celsius (sample 11), each at a rate of about 5 degrees Celsius per minute. Sample 6 is subsequently subjected to each of the other temperature profiles. The resultant product pellets are represented in Samples 6-11, which have the compositional distribution indicated in Table 3. Additional samples 12 et seq. have the compositional makeup as indicated in Table 3, and are subject to the temperature profile of Sample 7 (850 degrees Celsius) and are prepared in the same manner as the rest of the Samples in the this example.
Table 3 shows that the metal oxide varistor materials may perform relatively well, displaying a breakdown field of greater than about 1700 volts per millimeter and a good nonlinearity coefficient (α) of greater than about 75, but still having a relatively low sintering temperature.

TABLE 3

Composition				Samples	Sample	Sample	Sample
(Wt percent)	Sample 4	Sample 5	Sample 6	7-11	12	13	14

ZnO	85.5	94	94.69	94.69	95.0	84.0	94.0
Bi₂O₃	2	0.5	3	3	3.5	3	3
Sb₂O₃	3	1.4	1.5	1.5	0.2	3	0.1
Al₂O₃	3	—	0.01	0.01	0.1	—	—
SiO₂	3	2	—	—	0.1	0.5	1.0
Cr₂O₃	0.95	0.04	—	—	0.1	—	—
MnO	—	—	—	—	0.1	0.5	—
Mn₂O₃	—	0.6	—	—	0.1	—	—
MgO	0.05	—	—	—	0.1	1	—
Fe₂O₃	—	—	—	—	0.1	0.5	—
Co₂O₃	—	—	—	—	0.1	2.5	—
Co₃O₄	2.5	0.5	0.5	0.5	0.1	—	—
NiO	—	0.96	0.2	0.2	0.1	3	—
SnO₂	—	—	—	—	0.1	—	—
Li₂CO₃	—	—	0.1	0.1	0.1	1.5	0.9
LiBiO₃	—	—	—	—	0.1	—	1
CaO	—	—	—	—	0.1	0.5	—
Breakdown Field	>1800	>1800	>1850	—	—	—	—
(V/mm)
Nonlinearity	>75	>80	>75	—	—	—	—
coefficient (α)

In the specification and claims, reference will be made to a number of terms have the following meanings. The singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise. Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term such as “about” is not to be limited to the precise value specified. In some instances, the approximating language may correspond to the precision of an instrument for measuring the value. Similarly, “free” may be used in combination with a term, and may include an insubstantial number, or trace amounts, while still being considered free of the modified term.
As used herein, the terms “may” and “may be” indicate a possibility of an occurrence within a set of circumstances; a possession of a specified property, characteristic or function; and/or qualify another verb by expressing one or more of an ability, capability, or possibility associated with the qualified verb. Accordingly, usage of “may” and “may be” indicates that a modified term is apparently appropriate, capable, or suitable for an indicated capacity, function, or usage, while taking into account that in some circumstances the modified term may sometimes not be appropriate, capable, or suitable. For example, in some circumstances an event or capacity can be expected, while in other circumstances the event or capacity cannot occur—this distinction is captured by the terms “may” and “may be”.
Reactants and components referred to by chemical name or formula in the specification or claims hereof, whether referred to in the singular or plural, may be identified as they exist prior to coming into contact with another substance referred to by chemical name or chemical type (e.g., another reactant or a solvent). Preliminary and/or transitional chemical changes, transformations, or reactions, if any, that take place in the resulting mixture, solution, or reaction medium may be identified as intermediate species, master batches, and the like, and may have utility distinct from the utility of the reaction product or final material. Other subsequent changes, transformations, or reactions may result from bringing the specified reactants and/or components together under the conditions called for pursuant to this disclosure. In these, other subsequent changes, transformations, or reactions the reactants, ingredients, or the components to be brought together may identify or indicate the reaction product or final material.
The embodiments described herein are examples of articles, compositions, and methods having elements corresponding to the elements of the invention recited in the claims. This written description may enable those of ordinary skill in the art to make and use embodiments having alternative elements that likewise correspond to the elements of the invention recited in the claims. The scope of the invention thus includes articles, compositions and methods that do not differ from the literal language of the claims, and further includes other articles, compositions and methods with insubstantial differences from the literal language of the claims. While only certain features and embodiments have been illustrated and described herein, many modifications and changes may occur to one of ordinary skill in the relevant art. The appended claims cover all such modifications and changes.

Claims

1. A method, comprising:

contacting a transition metal oxide, a sintering additive, and a grain growth inhibitor additive to form a mixture, wherein the transition metal oxide comprises particles that have an average diameter less than about 1 micrometer; and

sintering the mixture at a temperature profile that is sufficiently high that a sintered mass is formed from the mixture, and the thermal profile is less than about 1050 degrees Celsius.

2. The method of claim 1, wherein treating the mixture comprises exposure to a sinter temperature that is in a range from about 700 degrees Celsius to about 1050 degrees Celsius.

3. The method of claim 1, further comprising calcining the transition metal oxide and the sintering additive together before forming the mixture, and the calcining comprises heating to a temperature of about 400 degress Celsius.

4. The method of claim 1, further comprising contacting the sintered mass with an electrically conductive material, and forming a electrical connection therebetween.

5. The method as defined in claim 4, wherein the electrically conductive material has a melting point that is less than 1050 degrees Celsius, and forming the electrical connection comprises forming the electrically conductive material into an electrode.

6. The method as defined in claim 1, further comprising selecting the sintering additive to comprise one or more of lithium, antimony, bismuth, cobalt, manganese, or silicon.

7. The method as defined in claim 6, further comprising selecting the sintering additive to comprise one or both of LiBiO₃, or Li₂CO₃.

8. The method as defined in claim 1, further comprising selecting the transition metal oxide to be present in an amount that is greater than about 80 percent by weight, based on the total weight of the the sintered mass.

9. The method as defined in claim 1, further comprising selecting the amount of the sintering additive to be less than about 15 percent by weight, based on the total weight of the the sintered mass.

10. The method as defined in claim 1, further comprising selecting the grain growth inhibitor additive to comprise one or more of SiO₂, Sb₂O₃, CaO, Al₂O₃, MgO, or Fe₂O₃.

11. The method as defined in claim 1, further comprising selecting the grain growth inhibitor additive to be present in an amount that is less than about 10 percent by weight, based on the total weight of the the sintered mass.

12. The method as defined in claim 1, further comprising adding a a grain boundary additive to the mixture prior to sintering.

13. The method as defined in claim 12, further comprising selecting the grain boundary additive to comprise Co₃O₄, Cr₂O₃, Bi₂O₃, Pr₂O₃, NiO, or SnO₂.

14. The method as defined in claim 12, further comprising selecting the amount of the grain boundary additive to be less than about 10 percent by weight, based on the total weight of the the sintered mass.

15. The method as defined in claim 1, wherein the sintered mass comprises a plurality of cores, and the temperature profile and a an average particle size of the transition metal oxide are selected to form an average distance from one core to an adjacent core in the plurality of cores is less than about 1 micrometer after sintering.

16. The method as defined in claim 15, wherein the average diameter of the cores is less than about 1 micrometer.

17. The method as defined in claim 15, wherein the cores are separated from each other by a grain boundary layer, and a mean value for a thickness of the grain boundary layer is less than 50 nanometers.

18. The method as defined in claim 17, wherein the cores each define a grain boundary in the sintered mass, and an average distance from a grain boundary of one core to a grain boundary of an adjacent core in the sintered mass is less than about 1 micrometer.

19. The method as defined in claim 12, wherein selecting the grain boundary additive results in the average thickness of the grain boundary layer being less than about 400 nanometer.

20. The method as defined in claim 1, further comprising subjecting the sintered mass to an electrical potential, and the sintered mass exhibits a dielectric strength or breakdown field of greater than about 0.5 kV/mm.

21. The method as defined in claim 20, wherein the sintered mass has a non-linearity coefficient (α) of greater than about 25.

22. The method as defined in claim 1, further comprising forming the sintered mass with a homogenous microstructure.

23. A method, comprising:

contacting a transition metal oxide with a sintering additive to form a premix, wherein the transition metal oxide comprises particles that have an average diameter less than about 1 micrometer;

calcining the premix, wherein the calcining comprises heating to a temperature of about 400 degress Celsius to provide a calcined mass;

contacting the calcined mass with a grain growth inhibitor additive to form a mixture; and

24. A method, comprising:

sintering the mixture at less than about 1050 degrees Celsius to form a sintered mass, and while sintering forming an electrode from an electrically conductive material.