Patent Description:
Size reduction of power transmission and conversion devices is demanded with the enhanced capacity of power substations and development of underground stations. As a consequence, gas-insulated switchgear (GIS) surge varistors of smaller size and/or simpler structure are required to follow the trend and to reduce the consuming of SF<NUM> and housing materials. Such requirements call for a new generation of the key components, metal oxide varistors (MOV), of which the height should be appreciably reduced for a given protection voltage.

To fulfill the height reduction of MOVs several properties of the ceramic material, of which the MOV is made, have to be improved.

<CIT> discloses a ceramic material with ZnO as the main component and comprising additives which may include Bi<NUM>O<NUM>, Sb<NUM>O<NUM>, CO<NUM>O<NUM>, Mn<NUM>O<NUM>, NiO, Y<NUM>O<NUM>, and Al<NUM>+.

Objects of the present invention are to provide a ceramic material with improved properties to be used in a varistor and to provide a varistor containing ceramic body made of such a ceramic material. Further objects are to provide methods of preparing the ceramic material and of preparing a varistor.

These objects are fulfilled with a ceramic material according to the independent claim <NUM>, with a varistor according to independent claim <NUM> and with methods according to claims <NUM> and <NUM>. Further embodiments are subject of dependent claims.

According to the invention, a ceramic material is provided comprising ZnO as a main component, and additives comprising an Al<NUM>+-containing solution, a Ba<NUM>+-containing solution, Bi<NUM>O<NUM>, Sb<NUM>O<NUM>, Co<NUM>O<NUM>, Mn<NUM>O<NUM>, NiO, Y<NUM>O<NUM>, and optionally Cr<NUM>O<NUM>.

"Ceramic material" is to be understood as a composition of components that is prepared in a way that it only has to be sintered to become a ceramic. Bodies formed of the ceramic material may be called green bodies. When the ceramic material is sintered a ceramic is formed which has properties dependent on the composition of the ceramic material.

The additives Al<NUM>+-containing solution and Ba<NUM>+-containing solution are to be understood as starter materials being added to the main component ZnO. Alternatively, these additives can be named Al<NUM>+ and Ba<NUM>+, respectively, which are added in form of an Al<NUM>+-containing solution and a Ba<NUM>+-containing solution when preparing the ceramic material.

The content of the additives in the ceramic material is according to one embodiment ≤ <NUM> mol%. It is to be understood that the content of all additives together is ≤ <NUM> mol%. This is reduced content of additives compared to ceramics of the prior art having typically additives with an amount of <NUM> to <NUM> mol%. The low content of additives in the ceramic material results in a low content of secondary phases in the sintered ceramic which enhances the effective ZnO phase and ZnO-ZnO grain boundaries leading to higher volume efficiency of the varistor ceramics.

It is described that, in the ceramic material c<NUM> is the equivalent content of Co in Co<NUM>O<NUM>, m is the equivalent content of Mn in Mn<NUM>O<NUM>, s is the equivalent content of Sb in Sb<NUM>O<NUM>, c<NUM> is the equivalent content of Cr in Cr<NUM>O<NUM>, a is the content of Al<NUM>+, y is the equivalent content of Y in Y<NUM>O<NUM>, b<NUM> is the equivalent content of Bi in Bi<NUM>O<NUM>, n is the equivalent content of Ni in NiO, b<NUM> is the content of Ba<NUM>+, and z is the content of ZnO. According to the invention it is: <MAT> <MAT> <MAT> <MAT> <MAT> <MAT> <MAT> <MAT> , and <MAT>.

In the above relationships, b<NUM> corresponds to the content of Bi<NUM>O<NUM>, s corresponds to the content of Sb<NUM>O<NUM>, c<NUM> corresponds to the content of Co<NUM>O<NUM>, m corresponds to the content of Mn<NUM>O<NUM>, n corresponds to the content of NiO, c<NUM> corresponds to the content of Cr<NUM>O<NUM>, and y corresponds to the content of Y<NUM>O<NUM>.

Further, (c<NUM>+5c<NUM>+<NUM>+4y-m-250a)(<NUM>-z)/b<NUM> may be called composition factor F and it applies <NUM> ≤ F ≤ <NUM>. The relationship of the contents of the different additives in the ceramic material is responsible for the grain size control and the formation of grain boundary potential during sintering of the ceramic material in order to achieve an ultra-high varistor gradient (E1mA) of the sintered ceramic from <NUM> V/mm inclusive to <NUM> V/mm inclusive. The varistor gradient is the characteristic varistor voltage per mm.

The correlation between the contents of additives and varistor gradient enables the minimization of varistor inactive phases in the sintered ceramic, e.g. spinel phases Zn<NUM>Sb<NUM>O<NUM>, by simultaneously and properly changing the related elements for a desired varistor gradient. As a result, the volume efficiency of the sintered ceramic can be enhanced with more effective ZnO-ZnO grain boundaries.

It is described that at least one compound may be chosen from the group containing metal oxides, metal carbonates, metal acetates, metal nitrides and mixtures thereof. The at least one compound may be chosen from the group containing Bi<NUM>O<NUM>, Sb<NUM>O<NUM>, Co<NUM>O<NUM>, Mn<NUM>O<NUM>, NiO, Y<NUM>O<NUM>, and Cr<NUM>O<NUM>. For example, all compounds containing a metal element may be metal oxides each containing a different metal element.

Further, according to one embodiment the Al<NUM>+-containing solution and the Ba<NUM>+-containing solution may be solutions chosen from a group comprising nitrides, acetates, hydrates, and mixtures thereof. For example, the Ba<NUM>+-containing solution may be a solution of Ba(CH<NUM>COO)<NUM> and the Al<NUM>+-containing solution may be a solution of aluminium nitrate Al(NO<NUM>)<NUM>. The content of Ba<NUM>+ in the ceramic material is adjusted to reduce the high temperature power loss and/or leakage current of the ceramic made of the ceramic material, and to improve the non-linearity of the I/V curve of a varistor containing a ceramic body made of the ceramic material.

According to one embodiment the ceramic material has a sintering temperature of between <NUM> inclusive and <NUM> inclusive. This reduced sintering temperature requires less energy. This is advantageous in view of environment protection and enables a fast production of varistor devices containing ceramics made of the ceramic material. Further, the evaporation of the Bi<NUM>O<NUM> is thermodynamically depressed leading to less evaporation of Bi<NUM>O<NUM> while reducing possible composition deviations and possible inhomogeneity induced by sintering.

Homogeneity of the ceramic leads to homogeneity of the current distribution in a varistor made of the ceramic material during operation being crucial for the energy capacity of a varistor device.

Thus, the ZnO based ceramic material according to the above mentioned embodiments may be used for metal oxide varistors (MOVs) which can be used in gas isolated arresters (GIS). The ultrahigh varistor gradient of the ceramics made of the ceramic material enables a miniaturization and design simplification of the arrester devices. Further, the ceramics show reduced high temperature power loss preventing the thermal run-away even under worse heat dissipation conditions.

Further, a varistor is provided comprising a ceramic body containing a sintered ceramic material according to the above-mentioned embodiments. Due to the composition of the ceramic material the varistor can be called a metal oxide varistor (MOV). The varistor may have a varistor gradient E1mA of between <NUM> V/mm inclusive and <NUM> V/mm inclusive. Thus, an ultra-high gradient of the varistor is provided. Such a varistor may, for example, be used in compact GIS arresters.

Due to use of a ceramic material according to one of the above mentioned embodiments in a varistor the varistor has several advantageous properties. For example, the quantity of secondary phases in the ceramic body can be well reduced due to the composition of the ceramic material.

In conventional varistor ceramics, the grain size is controlled by spinel phase (Zn<NUM>Sb<NUM>O<NUM>) which is actually not active in varistor responses. The formation of the spinel phase during sintering consumes quite a lot of ZnO out of the composition of the ceramic material (<NUM> mol Sb<NUM>O<NUM> corresponds to <NUM> mol ZnO). Therefore, for a composition containing <NUM> mol% Sb<NUM>O<NUM> about <NUM> mol% of the ZnO would be taken by the spinel phase and maximal <NUM> mol% of ZnO would be available for forming varistor grain boundaries in the final ceramic. A high content of the spinel phase also reduces the connectivity of the ZnO grain boundaries. As a consequence, the volume efficiency is strongly limited, especially when the higher varistor gradient has to be achieved by increasing the spinel phase to reduce the grain size.

Introduced Y<NUM>O<NUM> in the ceramic material can react with Bi<NUM>O<NUM>, Sb<NUM>O<NUM>, and a small quantity of ZnO (comparable to that of Sb<NUM>O<NUM>) during sintering and form very fine particles (less than one micron in diameter) which depress the grain growth more effectively than spinel. Thus, the ultra-high varistor gradient can be achieved with a reduced content of Sb<NUM>O<NUM>, e.g. as low as <NUM> mol%, in the ceramic material. The reduced content of Sb<NUM>O<NUM> (or spinel phase in the final ceramic) and of other additives as a whole (≤ <NUM> mol%) lead to a high volume efficiency of the varistor containing the ceramic made of the ceramic material.

Additionally, the ceramic body of the varistor made of the ceramic material has a desirable high temperature power loss PCOV at <NUM> in dependence of device design, and excellent steepness s7 (being the relation of the clamping gradient E10kA to the varistor gradient E1mA: E10kA/E1mA), especially a steepness s7 being ≤ <NUM>. These properties are also suitable for a ceramic of varistors to be used in GIS arresters with reduced size and worsened heat dissipation conditions.

Thus, the height reduction of the MOV can be fulfilled by use of the ceramic material as the varistor gradient is appreciably increased and at the same time the high temperature power loss of the MOV is decreased.

It is to be understood that features mentioned in context of the ceramic material apply also to the varistor and features mentioned in context of the varistor apply also to the ceramic material.

Further, a method of preparing the ceramic material according to one of the above-mentioned embodiments is provided. The method has the preparation steps of weighing, mixing and ball-milling a first part of additives, adding ZnO and a second part of additives, forming a homogenous slurry, and spray-drying the slurry to form a granule of the ceramic material. The first part of additives is Bi<NUM>O<NUM>, Sb<NUM>O<NUM>, Y<NUM>O<NUM>, CO<NUM>O<NUM>, optionally Cr<NUM>O<NUM>, Mn<NUM>O<NUM>, and NiO and the second part of additives is an Al<NUM>+-containing solution, and a Ba<NUM>+-containing solution.

It is also described that the first part of additives may be at least one compound containing a metal element, wherein the metal element is chosen from a group comprising Bi, Sb, Co, Mn, Ni, Y, and Cr.

The additives Bi<NUM>O<NUM>, Sb<NUM>O<NUM>, Y<NUM>O<NUM>, CO<NUM>O<NUM>, Cr<NUM>O<NUM>, Mn<NUM>O<NUM>, and NiO or, for example, other types of oxides, carbonates, acetates, nitrides with the equivalent quantity of metal elements are weighed, mixed and ball-milled, for example in water, to get a desired particle size distribution. The main component ZnO may be added in the form of powder and introduced together with, for example, an Al<NUM>+- and a Ba<NUM>+-containing solution in form of nitrides, acetates or hydrides into the system. For forming a homogenous slurry additional water and some organics such as, for example binders, dispersing agents, defoaming agents may be further introduced and a homogenous slurry of desirable viscosity and density can be formed. The spray-drying of the slurry is performed to get a granule of desired diameter, flowability and pressability for the subsequent process steps.

Further, a method of preparing a varistor is provided comprising the preparation steps: forming a ceramic body containing the ceramic material being prepared according to one of the above-mentioned embodiments and applying electrode layers, wherein the ceramic material is sintered at a temperature of between <NUM> inclusive and <NUM> inclusive to form the ceramic body.

The forming of the ceramic body comprises the further steps dry-pressing a granule of the ceramic material prepared with the method mentioned above, debindering the ceramic material and sintering the ceramic material, wherein the steps are performed before sintering.

For example, cylinder-shaped green parts of defined sizes formed from a granule of ceramic material made with the method mentioned above are provided, wherein the defined sizes may depend on the further characterization method. For characteristic properties as varistor gradient E1mA, clamping gradient E10kA, and leakage current densitiy JS, the dimension of the green parts may be <NUM> in diameter and <NUM> in thickness. For energy varistor demonstrations, the diameter may be <NUM> to <NUM> and the thickness may be <NUM>. The green parts may be dry-pressed from the granule followed by debindering in air at about <NUM> to remove the organic components. The parts may be then sintered at <NUM> to <NUM> for one to three hours to get a dense and uniform varistor ceramic body.

As electrode layers layers of for example Al or Ag may be applied on the top and bottom surface of the ceramic body. Al may be applied for example by a Schoop process, wherein molten metal droplets are sputtered on a solid surface to form an electrode. Ag may be applied for example by sputtering.

Additionally, on the side surfaces of the ceramic body isolation layers may be applied. For example, a high isolative glaze coating may be applied by spraying and tempering. The glaze coating may comprise glass. The tempering may be performed at a temperature of about <NUM>. The application of isolation layers may be performed before the application of electrode layers.

Embodiments of the ceramic material, the varistor and the methods of preparing the ceramic material and the varistor are further explained in the following by examples and figures.

Equal, similar or apparently equal elements have the same numbers or symbols in the figures. The figures and the proportions of elements in the figures are not drawn to scale. Rather, several elements may be presented disproportionately large for a better presentation and/or a better understanding.

<FIG> shows the cross-section of a varistor according to one embodiment. It contains the ceramic body <NUM>, electrode layers <NUM> and isolation layers <NUM>. For the preparation of the varistor first the ceramic body <NUM> is formed.

For this, solid state additive raw material as Bi<NUM>O<NUM>, Sb<NUM>O<NUM>, Y<NUM>O<NUM>, Co<NUM>O<NUM>, Cr<NUM>O<NUM>, Mn<NUM>O<NUM> and NiO (or, for example, other types of oxides, carbonates, acetates, or nitrides with the equivalent quantity of metal elements) are weighed, mixed and ball-milled in water to get the desired particle size distribution. The main component ZnO is then introduced in form of a powder together with the Al<NUM>+- and Ba<NUM>+-solution (in terms of nitrides, acetates or hydrates) into the system. Additional water and some organics (e.g. binder, dispersing agent, defoaming agent) are further introduced to form a homogeneous slurry of desirable viscosity, density, or solid content. A granule of desired diameter and size distribution, packing density, flowability and pressability is then produced by spray-dry method out of the slurry.

The size of the further formed cylinder-shaped green parts containing the granules of ceramic material depends on the further characterization method: for example, for electric characterization disk-shaped green parts of <NUM> in diameter and <NUM> in thickness are dry-pressed from the granule, followed by debindering in air at about <NUM>° to remove the organic components. The discs are then sintered at <NUM> for three hours to get a dense ceramic body. The top and bottom surfaces are metallized, for example with Ag by sputtering.

For energy varistors as shown in <FIG>, the side surface of the ceramic part is coated with a layer of high isolative glaze by spraying and tempering to form the isolation layer <NUM>. The top and bottom surfaces are ground to remove the contaminated glaze and to get desired height and surface quality. The top and bottom surfaces are fully metallized, for example with Al by Schoop-process.

In the following, several examples for ceramic materials and ceramics made thereof are shown.

The samples E01 to E43 described in table <NUM> explain the correlation between the contents of additives and varistor gradient E1mA under defined process conditions. Within the specified range of each component, including ZnO and other additives, the varistor gradient strongly depends on the relative content of the most relevant additive components Bi<NUM>O<NUM>, Sb<NUM>O<NUM>, Y<NUM>O<NUM>, Co<NUM>O<NUM>, Cr<NUM>O<NUM>, Mn<NUM>O<NUM>, NiO and Al<NUM>+. The dependence can be expressed by a linear correlation between E1mA and the composition factor F, wherein F is the function of the content of Co<NUM>O<NUM> (c<NUM>), Mn<NUM>O<NUM> (m), Sb<NUM>O<NUM> (s), Cr<NUM>O<NUM> (c<NUM>), Al<NUM>+ (a), Y<NUM>O<NUM> (y), Bi<NUM>O<NUM> (b<NUM>), and ZnO (z):<MAT>.

To get a desired varistor gradient, e.g. E1mA is between <NUM> V/mm inclusive and <NUM> V/mm inclusive, the content of the additives should be adjusted so that the factor F is between <NUM>, preferably <NUM> inclusive and <NUM> inclusive.

The samples E01 to E43 are prepared as mentioned with respect to <FIG>, wherein disk-shaped green parts of <NUM> in diameter and <NUM> in thickness are dry-pressed from the granule, followed by debindering in air at about <NUM> to remove the organic components. The disks are then sintered at <NUM> for three hours to get dense ceramic bodies.

For the characterization of the samples, the discs are metallized at the top and bottom surface with Ag by sputtering. The electric properties of the metallized parts are characterized and then normalized to an energy varistor of <NUM> in diameter and <NUM> in height for a fair comparison.

The varistor gradient E1mA is measured with a low DC current, which gives a current density of <NUM> mA over a ceramic disc of <NUM> in diameter (or about <NUM>µA/cm<NUM>). The clamping gradient E10kA is measured with <NUM>/<NUM> discharging wave, and gives a current density of <NUM> kA over a ceramic disc of <NUM> in diameter (or about <NUM> A/cm<NUM>). The steepness s7 is equal to E10kA/E1mA. The leakage current density at room temperature and at <NUM>°, JS, is measured under DC field of <NUM> E1mA.

Table <NUM> shows the composition of each example E01 to E43, wherein the contents of ZnO and the additives are given in mol%, the factor F and the varistor gradient E1mA. It can be seen that, for a factor F which is between <NUM> and <NUM> the varistor gradient is in the desirable range between <NUM> V/mm and <NUM> V/mm. If factor F is bigger or smaller than <NUM> and <NUM>, respectively, the desirable range of the varistor gradient cannot be achieved (examples E03, E06 to E08, E18, E23 and E24).

The dependence of the varistor gradient on the composition factor F is also shown in <FIG> where the correlation between E1mA and F can be clearly seen.

The expression of F reflects the effectiveness of a respective component affecting the varistor gradient which is actually determined by grain size and the grain boundary potential formed during sintering. The grain growth is mainly controlled by the formation and distribution of the secondary phases, e.g. spinel phase Zn<NUM>Sb<NUM>O<NUM>, and Y-Bi-rich phase as can be seen in <FIG>. In <FIG> the microstructure of the exemplary sample E15 ceramic is shown. A shows a Y-Bi-rich phase, B shows a Bi-rich liquid phase and C shows a spinel phase Zn<NUM>Sb<NUM>O<NUM>.

A spinel phase is characterized with a grain size of <NUM> to <NUM>, while the Y-Bi-rich phase has a much smaller size of submicron in diameter. As a consequence, the introduction of Y<NUM>O<NUM> brings about double effectiveness in comparison with that of Sb<NUM>O<NUM> in achieving the desired varistor gradient. The formation of spinel phase consumes much ZnO and is not favourable for high volume efficiency. By introducing <NUM> mol% Sb<NUM>O<NUM>, about <NUM> mol% of the ZnO would be taken by the spinel phase and the effective varistor volume fraction (ZnO grains) has to be reduced by so much. In contrast, the Y-Bi-rich phase grains have a small content of ZnO and have minor influence on the volume efficiency. Accordingly, ultra-high gradient varistor ceramics with reduced Sb<NUM>O<NUM> content (e.g. <NUM> mol% to <NUM> mol%) can be achieved by collaboratively varying the contents of other additives so that the factor F is within a certain scale (e.g. the compositions of E12, E22 and E33 to E42).

Samples E44 to E46 are listed in Table <NUM> together with samples E15 and E25, and show the influence of Ba<NUM>+-content on the high temperature leakage current JS and steepness characteristic s7.

The compositions listed in Table <NUM> all have the same amount of additives except the content of Ba<NUM>+. The contents of ZnO and the additives are given in mol%. The preparation process and electric characterization are the same as described for examples E01 to E43. The basic electric properties as a function of the Ba<NUM>+-content b<NUM> are plotted in <FIG>. Here, the content of Ba<NUM>+ b<NUM> is plotted against the high temperature leakage current JS, the steepness s7 and the varistor gradient E1mA. The circles in <FIG> are the respective measured values of the samples, the boxes are statistic representations of the scattering (e.g. median, mean, maximum, minimum etc. value, depending on the definition in the plotting). Apparently, a trace amount as low as <NUM>% of Ba<NUM>+ can effectively reduce the high temperature leakage current density JS, which is crucial for the energy capability in device operation. A further increase of b<NUM> leads to even lower leakage current, but the steepness s7 gets worse when the b<NUM> exceeds <NUM>%.

The samples E47 to E49, listed in Table <NUM> together with samples E10, E15 and E19, are prepared out of the same compositions as E10, E15 and E19 respectively except that the dimensions of the green parts before debindering and sintering are <NUM> in diameter and <NUM> in height. Thus, in Table <NUM>, the sample number (examples), the composition, the diameter D and the thickness T are listed together with the electric properties varistor gradient E1mA, steepness s7, high temperature leakage current Js and high temperature power loss Pcov.

The debindering and sintering conditions are the same as the small discs in examples E01 to E46. A layer of glass material is then sprayed on the side surface of the sample, followed by tempering at <NUM> to get a dense and highly isolative glaze coating. The parts are then ground on both main sides to desired thickness, e.g. <NUM>. Aluminum metallization is provided on the top and bottom surface for electric contacting, for example by Schoop-process. The basic electric properties are characterized (the high temperature power loss PCOV instead of JS is measured at <NUM> under a <NUM>-AC field of E10kA/<NUM> in amplitude) and compared with a small disc of examples E10, E15 and E19. Basically, the varistor gradient E1mA of the small discs could be well reproduced with a slight offset of about <NUM>%, while the steepness s7 drifts to lower values due to the size effect. Lower PCOV could be expected for materials of lower Js as both arise from the same physical effect (high temperature resistance).

Claim 1:
Ceramic material comprising
- ZnO as a main component, and
- additives comprising an Al<NUM>+-containing solution, a Ba<NUM>+-containing solution, Bi<NUM>O<NUM>, Sb<NUM>O<NUM>, Co<NUM>O<NUM>, Mn<NUM>O<NUM>, NiO, Y<NUM>O<NUM>, and optionally Cr<NUM>O<NUM>
wherein c<NUM> is the content of Co<NUM>O<NUM>, m is the content of Mn<NUM>O<NUM>, s is the content of Sb<NUM>O<NUM>, c<NUM> is the content of Cr<NUM>O<NUM>, a is the content of Al<NUM>+, y is the content of Y<NUM>O<NUM>, b<NUM> is the content of Bi<NUM>O<NUM>, n is the content of NiO, b<NUM> is the content of Ba<NUM>+, and z is the content of ZnO and it is <MAT> <MAT> <MAT> <MAT> <MAT> <MAT> <MAT> <MAT> , and <MAT>
wherein
(c<NUM>+5c<NUM>+<NUM>+4y-m-250a)(<NUM>-z)/b<NUM> = F and <NUM> ≤ F ≤ <NUM>.