ZINC OXID VARISTOR

Abstract

Zinc oxide varistor comprising zinc oxide (ZnO) as a major component, one or more types of additives selected as a grain boundary component from a group consisting of bismuth (Bi) and praseodymium (Pr), and one or more types of additives selected as a transition metal element from a group consisting of cobalt (Co), manganese (Mn), and nickel (Ni); wherein the zinc oxide has a crystallite size of 20 to 100 nm, determined by X-ray diffraction, a grain diameter of 20 to 110 nm, determined by a BET method, and an untamped density of 0.60 g / cm³ 3 or higher, and a tapped density of 0.80 g / cm³ 3 or higher.

Description

TECHNICAL AREA

[0001] The present invention relates to a zinc oxide varistor for protecting a circuit, for example, against lightning-induced overvoltage, etc. GENERAL STATE OF THE ART

[0002] Due to the rapid expansion of electronic devices in various sectors, the operating environments of the electronic components mounted on them have also changed considerably. For example, the revision of standards and the increasing number of applications in automobiles, industrial plants, and other fields have led to previously unforeseen increases in the performance and other requirements of electronic components. Consequently, electronic components capable of protecting sensitive electronic circuits from unexpected noise and impulses, such as overvoltages and pulse noise, are in high demand. Furthermore, due to the need to adequately consider the environmental influences of sulfurization, water condensation, and other factors, the provision of electronic components with not only high initial performance but also high long-term reliability is essential.

[0003] A varistor, conventionally described as a nonlinear resistive element, is used to protect a sensitive circuit from abnormal voltages such as various overvoltages and pulse noise in wide fields and applications, as mentioned above, to ensure operational stability and provide a countermeasure against ESD (electrostatic discharge). Furthermore, while many electronic components have been designed to save space and become thinner and shorter, the varistor is no exception. While miniaturization and cost reduction of the varistor are desirable, further reduction of its size and footprint has reached its limits under current conditions.

[0004] The electrical properties and reliability of the resulting varistor vary considerably depending on the combination and dosage of additives used. For example, grain growth during sintering can fluctuate due to the mixing ratio of the base materials added. This results in significant differences in the clamping voltage, a fundamental property of the varistor, and its ability to protect the circuit during large overvoltages.

[0005] In the following, ceramics and zinc oxide (ZnO) are described as sintering elements.

[0006] Zinc oxide exhibits properties such as a higher zinc vapor pressure and easier grain growth than other ceramic powders like aluminum oxide and zirconium oxide. The base powder has long been widely used as a white pigment, with established French and German manufacturing processes, and high-quality powders are available at low cost. However, the smallest grains of these powders are only about 0.3 to 0.6 µm in size, and densification of the base material for a sintering element at low temperatures is not to be expected, resulting in large sintered grains.

[0007] In recent years, powders with a particle size of 0.3 µm or less, produced using a wet process, primarily for cosmetics, have become available at relatively low cost. While it is important that the zinc oxide powder of the base material has a small particle size to enable low-temperature sintering without melting or the addition of sintering agents, there is no proposed effective means of controlling particle growth, which is one of the problems addressed by the present invention. The problems of the present invention are explained below, primarily based on the patent documents.

[0008] In patent document 1, zinc oxide, obtained by dropwise addition of a carbon-alkaline agent to water-soluble zinc carboxylate and monitoring until a constant pH is reached, forms tubular secondary particles from the aggregating primary particles. These secondary particles are intended for cosmetics and exhibit excellent UV shielding performance and transparency. However, the tapped density is low, and grain growth is significant due to the tubular or precursor-like markings. While it provides the necessary coverage for cosmetic applications, it is unsuitable as a base material for a sintered body. More specifically, acetic acid is added to and dissolved in an aqueous zinc chloride solution, and a sodium carbonate solution is added dropwise until a pH of 8 is reached.After the resulting precipitate has been passed through a filter and rinsed with water, it is dried for three hours and heat-treated at 400°C, resulting in a zinc oxide powder.

[0009] Patent document 2 relates to a basic zinc chloride in flake form with excellent covering properties. While this document provides a technology with excellent control of crystallinity, shape, and size, a significant amount of chlorine remains even after heat treatment to produce zinc chloride. Particularly with regard to aspect control, it is unsuitable as a sintering base material due to the large flake form with an aspect ratio of 10 or 15, the ease with which it sinters into slab form during dechlorination and dewatering, the size of the sintered grains, and the size of the voids at the time of sintering.

[0010] Patent document 3 relates to the production of a thermistor intended to improve performance as an exhaust gas temperature sensor for automobiles. This thermistor is produced by wet synthesis (spray pyrolysis) of a precursor, which enables both a uniform composition and a dense structure of a thermistor sintered element or a multiple oxide, and by heat treatment of the resulting powder with an average grain diameter of 30 to 50 nm. This heat treatment leads to an improvement in tapped density through grain growth, resulting in an average grain diameter of 0.1 to 1 µm. The use of grown grains can reduce the amount of binder required, yielding a dense sintered body with a nearly net-like structure. However, a lower sintering temperature and precise control of grain growth are not achievable with this method.

[0011] Patent document 4 relates to the production of granulated powder with high sphericity and provides a high-fill-rate filler for use in greases and paints. The production of the filler particles involves using a surfactant or binder in an organic solvent to create a suspension without specifying the primary zinc oxide particles to be used. The suspension is then dried using a spray dryer, resulting in a sphericity (major axis / minor axis) of 1.00 to 1.10 and a median diameter (D50) of 20 to 100 µm. Furthermore, the D90 / D10 size ratio of 2.8 or less results in very few extremely large particles, leading to an improved fill rate, a reduced angle of repose, and the provision of an excellent filler. However, the granulated powder obtained by this process is unsuitable as a sintering material.This is because, during sintering, the spherical and particulate powder (filler) shrinks, forming large cavities. These cavities can be reduced, but not eliminated, by sintering at high temperatures over a longer period.

[0012] Patent document 5 provides zinc oxide powders and zinc oxide multiple oxide powders for a sputtering target. The sputtering target requires high density, high thermal conductivity, and a uniform structure. In this technology, capsule HIP (hot isostatic pressing and sintering) is used as the sintering process for compaction, and, as a resulting problem, the capsule filling rate (base powder tapped density / theoretical density) is set to 50% or higher. A powder with a tapped density of 2.8 g / cm³ 3The process, achieved by sintering zinc oxide powder with a tapped density of less than 50% at 900 to 1400°C in air, is used as a means of solving the problem. While it is the same process as in patent document 3, since the method for improving the tapped density is a heat treatment, sintering with the capsule HIP process further prevents the volatilization of zinc oxide and also allows for a lower sintering temperature than sintering in air. It can be called a technology that achieves density, high strength, and low grain growth. However, this process provides a sintered material under the conditions of the capsule HIP process.

[0013] In non-patent document 1, petal-shaped zinc oxide with both high UV protection and high transparency is produced by titration under conditions of high temperature and constant pH. In this process, card-shaped precursor basic zinc carbonates combine and grow into petal shapes. When this is converted to zinc oxide by heat treatment, its shape is retained. However, because seed crystals become large and grain growth increases significantly, a uniform sintered body cannot be obtained. Specifically, a zinc chloride solution and an alkaline solution (a liquid mixture of sodium carbonate and sodium hydroxide) are added dropwise to water, which is maintained at 60°C to meet the requirement of a constant pH. After the resulting precipitate is passed through a filter and rinsed with water, it is dried, and the dry mass is sintered at 400°C, yielding zinc oxide. State-of-the-art documents, patent documents Patent Document 1: JP 2007-8805A Patent Document 2: JP 2015-038014A Patent Document 3: JP 2003-119080A Patent Document 4: JP Patent No. JP S56 17 410 B2 Patent Document 5: JP 2013-189369A Non-patent documents

[0014] Non-Patent Document 1: State-of-the-Art Research and Prospective of Zinc Oxide", 3. Particles, KATSUYAMA Tomoyuki (CMC Publishing CO., LTD.), published on January 31, 2011.

[0015] DE 698 23 042 T2 describes a process for producing varistors based on nanocrystalline powders produced by intensive mechanical grinding. In this process, a consolidation treatment includes sintering, the consolidation treatment being carried out under temperature and time conditions selected to simultaneously achieve a zinc oxide grain size of less than 3 microns and low porosity, with the sintering temperature being between 800°C and 1200°C and the treatment time at this sintering temperature being less than 2.5 hours.

[0016] JP S63-224303A describes a process for the production of ZnO varistors in which an aqueous solution of zinc salt and dopants (including Bi, Co, Mn, Cr, Sb, SiO2) is mixed with an amine solution (e.g., diethylamine) to precipitate at pH 8-10. The precipitate is washed, dried, calcined at 200-700 °C, then pulverized, pressed, and sintered in air / O2 at 1100-1300 °C.

[0017] JP 2001 - 220 136 A describes the production of transparent, UV-shielding fine-grained ZnO by introducing CO2 into a dilute aqueous ZnO slurry (≤ 10 wt%), resulting in basic zinc carbonate. The slurry is dried directly without solid / liquid separation (fluidized bed, medium fluidized bed, flash, or spray drying). Subsequently, the dried basic zinc carbonate is thermally decomposed (preferably at ~200–500 °C) into fine ZnO particles. Optionally, the slurry is mechanically concentrated before drying (e.g., to 20–50 wt%).

[0018] US 2010 / 0136337A1 describes high-voltage ZnO varistor powder with pronounced nonlinearity: main component ZnO (≥ 90 mol%); minor components (calculated as oxides / ions): Bi₂O₃ 0.3–1.5 mol%, Co₂O₃ 0.3–2.0 mol%, MnO₂ 0.3–3 mol%, Sb₂O₃ 0.5–4 mol%, NiO 0.5–4 mol%, Al 3+ 0.0005–0.02 mol%. Spherical powder with D50 = 20–120 µm, bulk density ≥ 2.5 g / cc, for field-control composites by dispersion in a polymer matrix. Additional requirements: particle size deviation ((D86–D16) / 2) ≥ 15 µm and mean ZnO primary crystal diameter ≤ 10 µm.

[0019] US 8 399 092 B2 describes high-density ZnO large particles for heat-dissipating (exogenous) fillers, among others, in resins, fats, coatings, as well as in rubber, pigments, electronics (ferrites, varistors), medicine and cosmetics: density ≥ 4.0 g / cm³ 3 , D50 = 17-10,000 µm, tap density ≥ 3.10 g / cm 3Preferably ≥ 85 wt% ZnO; as impurity metal oxides < 15 wt% of Mg, Ca, Ni, Co, Li, Na, K, Cu. Aspect ratio 1.00–1.10 (≥ 90% of particles ≤ 1.10) for nearly spherical morphology. Preparation: Granulation by mixing a ZnO source with halogen-free organic / inorganic acids or bases or their salts, followed by calcination. DISCLOSURE OF INVENTION Problem that the invention is intended to solve

[0020] As described above, a varistor has the ability to acquire its essential properties through the addition of various additives and the control of grain growth and the degree of sintering. However, it is difficult to achieve a varistor with ideal, outstanding basic properties. It is impossible to alter the properties of the zinc oxide base material itself, which constitutes approximately 90% of the varistor base material and is essential for the varistor's properties. Typically, the properties have been ensured through the interaction of the zinc oxide base material with various additives.

[0021] In general, zinc oxide exhibits properties such as a higher zinc vapor pressure and easier grain growth compared to other ceramic powders like aluminum oxide and zirconium oxide. Most zinc oxide is produced using the aforementioned French process, resulting in a grain size of approximately 0.3 to 0.6 µm, and contains numerous deformations and anisotropic components within the crystallites. Therefore, existing varistors utilizing these materials suffer from abnormal grain growth and unstable grain shape, which are generated during the sintering process. Furthermore, regarding tolerance to overvoltage pulses, such as those from lightning strikes, which is a crucial characteristic of a varistor, load concentration occurs due to grain irregularities when a high-voltage pulse is applied, leading to a significant degradation of its properties.

[0022] As a countermeasure to these problems, attempts have been made to improve performance by developing an additive composition and manufacturing process for the conventional varistor, but sufficient performance has not yet been ensured.

[0023] In view of these problems, the present invention aims to provide a highly reliable zinc oxide varistor that can be built compactly and cost-effectively. Means to solve the problems

[0024] As a means of achieving the aforementioned goal and solving the problems described above, a zinc oxide varistor according to the present invention is characterized by the inclusion of the following: zinc oxide (ZnO) as a major component, one or more types of additives selected as a grain boundary-forming component from the group consisting of bismuth (Bi) and praseodymium (Pr), and one or more types of additives selected as a transition metal element from the group consisting of cobalt (Co), manganese (Mn), and nickel (Ni), wherein the zinc oxide has a crystallite size of 20 to 100 nm, determined by X-ray diffraction, a grain diameter of 20 to 110 nm, determined by a BET method, and an untamped density of 0.60 g / cm³. 3 or higher and a tapped density of 0.80 g / cm³ 3 or higher.

[0025] For example, the zinc oxide varistor described above is characterized by the addition of one or more types of donor elements consisting of boron (B) and gallium (Ga).

[0026] The zinc oxide varistor according to the present invention is a zinc oxide powder used for the production of, for example, a sintered body, and is characterized in that an amount of aluminium (Al), specified by the expression (I) below, of 20 ppm or more and 2 mol% or less in a molar ratio is added to zinc (Zn) as a donor element; {nAl / (nZn+nAl)}×100 in which n A1 the amount of Al in the zinc oxide powder is called n Zn the amount of Zn in the zinc oxide powder and the unit of n Zn and n A1 Mol is.

[0027] Furthermore, this is characterized in that the aluminum-doped zinc oxide, which is produced by adding the aluminum (Al), is obtained by heat treatment at a temperature of 250°C or higher, a basic zinc carbonate, which is a carbonate hydrate and contains aluminum, which is produced by precipitation reactions of an aluminum salt, a zinc salt, a carbonate, and an alkali. It is further characterized in that the carbonate hydrate contains a basic zinc carbonate, specified by the following expression (1); M 4-6 (CO3) 1-3 (OH) 6-7 · nH2O (1) wherein M Zn 1-x Al x Let x be a number of 2×10 -5 denotes up to 0.02 and n denotes a number from 0 to 2.

[0028] Furthermore, it is characterized, for example, by the fact that either the powder of aluminum-containing zinc oxide is formed as is, or either after pulverization using a bead mill or granulation using a spray dryer, it is formed and sintered at a temperature of 1200°C or lower, resulting in a zinc oxide sintered body.

[0029] As a means of solving the problems described above, a zinc oxide varistor manufacturing process according to the present invention is characterized by the inclusion of the following steps: preparing a basic zinc carbonate slurry, which is a first precursor of a zinc oxide (ZnO) powder; obtaining from the first precursor a basic zinc carbonate dry powder, which is a second precursor of zinc oxide (ZnO) powder; heat-treating the second precursor to obtain zinc oxide (ZnO); preparing mixed materials resulting from the addition of predetermined additives to the zinc oxide (ZnO); and forming a varistor element for a zinc oxide varistor from the mixed materials.

[0030] For example, in the zinc oxide varistor manufacturing process, the zinc oxide varistor is characterized by the inclusion of: zinc oxide (ZnO) as a major component, one or more types of additives selected as a grain boundary-forming component from a group that includes bismuth (Bi) and praseodymium (Pr), and one or more types of additives selected as a transition metal element from a group that includes cobalt (Co), manganese (Mn), and nickel (Ni); wherein the zinc oxide has a crystallite size of 20 to 100 nm, determined by X-ray diffraction, a grain diameter of 20 to 110 nm, determined by a BET method, and an untamped density of 0.60 g / cm³ 3 or higher, and a tapped density of 0.80 g / cm³ 3 or higher.

[0031] For example, the manufacturing process of the zinc oxide varistor is characterized by the further addition of one or more types of donor elements, including boron (B) and gallium (Ga).

[0032] The manufacturing process of a zinc oxide varistor according to the present invention is further characterized in that an amount of aluminum (Al) 20 ppm or more and 2 mol% or less is added to zinc (Zn) as a donor element in a molar ratio. It is further characterized, for example, by the fact that the aluminum-doped zinc oxide produced by adding the aluminum (Al) is obtained by heat-treating at a temperature of 250°C or higher, or by a carbonate hydrate produced by precipitation under reaction conditions in an aqueous aluminum salt solution, an aqueous zinc salt solution, an aqueous carbonate solution, and an aqueous alkali solution.

[0033] Furthermore, it is characterized by the fact that the carbonate hydrate contains a basic zinc carbonate, specified by the following expression (1); M 4-6 (CO3) 1-3 (OH) 6-7 · n H2O (1) wherein M Zn 1-x Al x Let x be a number of 2×10 -5 denotes up to 0.02 and n denotes a number from 0 to 2.

[0034] Even further, it is characterized, for example, by the fact that either the powder of aluminum-containing zinc oxide is formed as is, or either after pulverization using a bead mill or granulation using a spray dryer, it is formed and sintered at a temperature of 1200°C or lower, resulting in a zinc oxide sintered body. Results of the invention

[0035] According to the present invention, a zinc oxide varistor capable of regulating variation in varistor voltage due to the uniform size of the sintered grains in a zinc oxide sintered body, and exhibiting excellent surge current resistance and low clamping voltage while maintaining high density and high electrical conductivity, can be provided. Furthermore, controlling the amount of aluminum added to the zinc oxide and the curing temperature allows for control of the sintered grain size and suppression of variation. BRIEF DESCRIPTION OF THE DRAWINGS The Fig. Figure 1 is a SEM image of a zinc oxide powder of synthesis example 1 according to the present invention; The Fig. Figure 2 is a SEM image of a zinc oxide powder of synthesis example 4 according to the present invention; The Fig.Figure 3 is a graph showing the relationships between the tapped density and the crystallite size of the zinc oxide powders from working example 1, synthesis example 2 and synthesis example 4; The Fig. Figure 4 is a flowchart showing the zinc oxide varistor manufacturing steps in time series according to an embodiment of the present invention; The Fig. Figure 5 is a diagram showing the evaluation results of the varistor voltage and the overvoltage withstand capability of the zinc oxide varistor according to the embodiment; and The Fig. Figure 6 is a diagram that provides the evaluation results of the varistor voltage and the clamping voltage of the zinc oxide varistor according to the embodiment. DESCRIPTION OF THE EXECUTION FORMS

[0036] One embodiment according to the present invention is described in detail below with reference to accompanying drawings and tables. 1. Zinc oxide powder [Configuration of zinc oxide powder]

[0037] (1) A zinc oxide powder used for a zinc oxide varistor according to the present invention has a crystal size of 20 to 100 nm, as determined by X-ray diffraction, a particle diameter of 20 to 110 nm, as determined by a BET method, and an untamped density of 0.60 g / cm³ 3 or higher and a tapped density of 0.80 g / cm³ 3 or higher.

[0038] Here, the untamped density is determined by measuring the mass at the moment the zinc oxide powder is released into a stationary 100 ml container using a method specified in JIS R 9301-2-3. This mass is then divided by the volume of the container to obtain the untamped density.

[0039] As described later in the working examples and comparative examples, the zinc oxide powder used for the zinc oxide varistor according to the present invention is characterized by having a higher tapped density than that of the comparative examples obtained using conventional technology. The composition of the zinc oxide powder used according to the present invention allows for a high filling density and increased contact points between particles when forming a press-molded body or a body formed into a thick layer using a paste. This makes it possible to obtain a sintered body that exhibits low shrinkage and is dense even at a low temperature of 1000°C or less. Furthermore, grain growth is low when sintering is carried out at a high temperature of 1000°C or higher.The sintered body obtained through sintering has a small sintered grain size and therefore high density and high strength.

[0040] (2) On the other hand, it has been proven, as will be described later, that zinc oxide powder used for the zinc oxide varistor according to the present invention, when added to the aluminum (Al) as a donor element, exhibits controlled grain growth and provides a uniform size of the sintered grain when the aluminum concentration is, for example, 20 mol ppm or higher and the baking or firing temperature is, for example, 1200°C or lower. Furthermore, when the aluminum concentration is less than 20 mol ppm and the baking temperature is higher than 1200°C, an increase in the size of the sintered grain is observed. [Zinc oxide powder manufacturing process]

[0041] The zinc oxide powder production process according to the present invention is not subject to any specific limitations. However, there is a suitable process for obtaining the zinc oxide powder of the present invention (also expediently referred to below as the 'powder production process of the present invention') by heat-treating an aluminum-containing, basic zinc carbonate, which is a carbonate hydrate produced by precipitation involving reactions of, for example, an aluminum salt, a zinc salt, a carbonate, and an alkali, at a temperature of 250°C or higher.

[0042] The zinc oxide powder obtained by the powder-making process of the present invention, for example, has a higher uncompacted density and tapped density than zinc oxide powders of the same crystallite size obtained by other processes, provided the Al content is the same (see evaluation 2 described later). While this is obvious if the heat treatment conditions for the precursor are high, dense secondary particles are formed by necking of primary particles, thereby increasing the uncompacted density and tapped density.

[0043] Furthermore, the use of the zinc oxide powder obtained using the powder manufacturing process of the present invention increases the density of the molded body and the sintered body (see Evaluation 3 and Evaluation 4, which will be described later).

[0044] Furthermore, the sintered body, using the zinc oxide powder obtained by the powder manufacturing process of the present invention, shows tendencies towards a smaller size of the sintered grain, a slight variation in this size and a high strength (see evaluation 4, which will be described later).

[0045] The zinc oxide powder obtained using the powder manufacturing process of the present invention contains aluminium that is homogeneously present in the powder particles, since it is produced from the aluminium-containing basic zinc carbonate (precursor), and thus presumably achieves the effects described above.

[0046] However, it must be added that individual particles of the zinc oxide powder are extremely tiny, making it virtually impossible to determine the condition of any aluminum trapped within it in order to specify it directly.

[0047] Specifying other components and characteristics based on the state of the aluminum requires many repetitions of the trial-and-error method and is therefore almost impractical.

[0048] The use of any compound consisting of an aluminum salt, a zinc salt, a carbonate and an alkali in an aqueous solution is preferred in the powder manufacturing process of the present invention.

[0049] It is preferred to induce precipitation-producing reactions by titrating, in particular, an aqueous zinc salt solution and an aqueous aluminum salt solution (preferably a mixed aqueous solution of zinc salt and aluminum salt) into an aqueous carbonate solution. During this titration, it is preferred to introduce an aqueous alkali solution into the aqueous carbonate solution in order to maintain the pH of the aqueous carbonate solution at a constant value (e.g., a value between pH 6 and 8).

[0050] The precipitation-inducing reactions yield a carbonate hydrate (basic zinc carbonate) as a precipitate. The precipitate is preferably stirred and solidified.

[0051] The stirring and hardening process is preferably one hour or more, more preferably five hours or more, and even more preferably ten hours or more, because the untamped density and the tapped density of the resulting zinc oxide powder increase. Stirring and hardening for 15 hours or more is particularly preferred.

[0052] In the case of a short stirring and hardening time, the primary particles, which are bound together to form flakes (e.g., see the one described later), should... Fig.2), which are characteristic of layered hydroxide, are easily obtained. In contrast, if the stirring and hardening time is long, the primary particles are expected to repeatedly collide with each other as a result of the stirring, losing their flake shapes and thus their grain shape, and in this way grain shapes (e.g., see the one described later) are expected to be lost. Fig. 1) easily obtained.

[0053] Note that although the duration of stirring and hardening depends on the concentration of the solution and the stirring force, the upper limit is not subject to any specific restriction and may be, for example, 32 hours or less, preferably 24 hours or less.

[0054] During the precipitation-inducing reaction, as well as during stirring and hardening, the temperature of the aqueous carbonate solution is preferably kept below 45°C. More preferably, it is 25°C or less.

[0055] The aluminum salt is not subject to any particular restrictions. However, it can preferably be, for example, aluminum nitrate, aluminum chloride, aluminum sulfate, or a hydrate thereof, etc.

[0056] While the zinc salt is not subject to any particular restriction, it may preferably be, for example, zinc nitrate, zinc sulfate, zinc chloride, zinc acetate or a hydrate thereof, etc.

[0057] The alkali is also not subject to any particular restrictions. However, it can preferably be, for example, sodium hydroxide, potassium hydroxide, or an aqueous ammonium solution, etc.

[0058] While the carbonate can be ammonium carbonate, sodium carbonate, sodium hydrogen carbonate (sodium bicarbonate), etc., ammonium carbonate is preferred for the reason that the unstamped density and the tapped density of the resulting zinc oxide powder are high.

[0059] It is preferred that the carbonate hydrate produced by the reactions causing the precipitation is an aluminum-containing basic zinc carbonate and more preferably contains the basic zinc carbonate specified by the following expression (1); M 4-6 (C03) 1-3 (OH) 6-7 · nH2O (1) wherein M Zn 1-x Al x Let x be a number of 2×10 -5 denotes up to 0.02 and n denotes a number from 0 to 2.

[0060] The basic zinc carbonate described by expression (1) above can be a basic zinc carbonate prepared by substituting aluminum for some of the zinc in hydrozincite (Zn5(CO3)2(OH)6 · 2H2O) and adding aluminum uniformly at a molecular scale. This type of basic zinc carbonate may also be referred to as hydrozincite below for simplicity.

[0061] The carbonate hydrate (basic zinc carbonate) produced by the precipitation-inducing reactions preferably has this type of hydrozincite as a major component. The major component is the component with the largest amount of the constitutive substances, preferably 50 wt% or more, more preferably 60 wt% or more.

[0062] The carbonate hydrate (basic zinc carbonate) obtained by the precipitation reactions is decarbonized and dehydrated by heat treatment at a temperature of 250°C or higher, yielding a zinc oxide powder.

[0063] There are cases where both removed amounts resulting from decarbonization and dehydration at the time of baking, when a subsequently described zinc oxide sintered body is obtained, increase if the heat treatment temperature is too low, leading to an impediment of sintering.

[0064] On the other hand, if the heat treatment temperature is too high, there is a risk that coupled particles, resulting from glued-together primary particles, may increase in size. Large coupled particles grow faster, forming larger sinter particles, a phenomenon known as Ostwald ripening, which results from non-uniform particle sizes within the sintered body.

[0065] From this perspective, a heat treatment temperature of 350°C to 420°C is preferred. However, it can easily be assumed that, due to the fact that the zinc oxide particles of the present invention are nanoscale, the growth and constriction of the primary particles will continue, leading to secondary particles that differ in density and strength and increase in both untamped and tapped density.

[0066] The powder manufacturing process of the present invention has been described as a preferred embodiment of the zinc oxide powder manufacturing process according to the present invention.

[0067] However, the process for producing the zinc oxide powder of the present invention is not limited to the powder production process of the present invention described above. Even if it is produced by a different process and pulverization, and the classification and particle size distribution, etc., are adjusted as required, it is still considered the zinc oxide powder of the present invention, as long as it falls within the scope of the present invention.

[0068] Note that another method may be a process for heat-treating a basic zinc carbonate that does not contain Al, which is produced by precipitate-generating reactions with a zinc salt, a carbonate and an alkali, for example, yielding a zinc oxide powder, and subsequently adding aluminum as an aqueous aluminum salt solution, etc., yielding an aluminum-containing zinc oxide powder. [Zinc oxide sintered bodies]

[0069] The zinc oxide sintered body of the present invention results from the sintering of the zinc oxide powder described above. Therefore, the zinc oxide sintered body of the present invention contains aluminum. The aluminum is preferably dissolved.

[0070] The zinc oxide sintered body of the present invention is obtained by baking the zinc oxide powder of the present invention described above. In particular, for example, either the zinc oxide powder of the present invention is shaped as is, or it is shaped either after pulverization using a bead mill or after granulation using a spray dryer, and the resulting shaped body is then baked or fired. This leads to the zinc oxide sintered body of the present invention.

[0071] The baking temperature, for example, is between 800°C and 1300°C inclusive. Furthermore, the baking temperature for the zinc oxide to which aluminum has been added is preferably 900°C or higher, more preferably 1000°C or higher. In addition, the baking temperature is preferably 1150°C or lower, more preferably 1100°C or lower.

[0072] The zinc oxide sintered body of the present invention is used as a ceramic component. In particular, it is suitablely used as: a sheet-like mass material; a baked thick-film object; a sputtering target requiring uniform density and particle size; a porous component, such as a gas sensor or filter (antibacterial filter to prevent the spread of Escherichia coli and the like, etc.), etc., for example. <arbeitsbeispiele>

[0073] The present invention is described in detail below by means of working examples. However, the present invention is not limited to these examples. <Synthesebeispiel 1 (Arbeitsbeispiele E und Vergleichsbeispiele 1)> (Synthesis)

[0074] Zinc nitrate 6-hydrate (manufactured by Kishida Chemical Co., Ltd.) is used as the zinc salt, aluminum nitrate 9-hydrate (manufactured by Kishida Chemical Co., Ltd.) is used as the aluminum salt, ammonium carbonate (manufactured by Kishida Chemical Co., Ltd.) is used as the carbonate, and 30 wt% sodium hydroxide (manufactured by Kishida Chemical Co., Ltd.) is used as the alkali.

[0075] A combination of zinc nitrate and aluminium nitrate, weighed to a total amount of 0.5 mol, is dissolved in 1L of pure water to produce a mixed aqueous solution of zinc nitrate and aluminium nitrate.

[0076] 0.5 L of a 0.4 molar aqueous ammonium carbonate solution are prepared in a 2 L beaker.

[0077] A pH electrode for regulating the pH value is inserted into the aqueous ammonium carbonate solution, and the mixed aqueous solution of zinc nitrate and aluminium nitrate is dripped into the aqueous ammonium carbonate solution at a rate of 1L / h, while the aqueous ammonium carbonate solution is stirred using a rotator at a rotation speed set to 700 rpm.

[0078] To prevent a decrease in the pH of the aqueous ammonium carbonate solution when adding the acidic aqueous solution of zinc nitrate and aluminum nitrate, 30 wt% sodium hydroxide is added to the aqueous ammonium carbonate solution using a liquid transfer pump controlled by a pH regulator (TDP-51, manufactured by Toko Kagaku Kenkyujo Co., Ltd.). This maintains a constant pH of 7.5 in the aqueous ammonium carbonate solution during the addition of the mixed aqueous solution of zinc nitrate and aluminum nitrate. As a consequence, a precipitation reaction produces a precipitate.

[0079] After the solution supply is complete, the solution is stirred and post-treated for 20 hours using a rotator set to the same rotation speed of approximately 700 rpm as during the precipitation reaction, yielding an aluminum-containing, basic zinc carbonate slurry.

[0080] With the aid of a cooling device, the temperature of the aqueous ammonium carbonate solution is always kept below 30°C during the precipitation-inducing reaction and the stirring and hardening.

[0081] The stirred and hardened slurry is separated into solid and liquid components using a suction filtration process to obtain the solid content. The resulting solid content is then rinsed to remove excess sodium, etc. Specifically, after the solid content is formed into a slurry using an appropriate amount of pure water, the resulting slurry is separated into solid and liquid components using a suction filtration process. This rinsing process is repeated four times.

[0082] The rinsed solids are vacuum-dried at 30°C for 20 hours using a vacuum dryer. The result is a dried powder of aluminum-containing, basic zinc carbonate or a precursor of zinc oxide powder.

[0083] According to synthesis example 1 (working examples E and comparative examples 1), a synthesis is carried out such that the molar ratio (Al / Zn) of aluminium and zinc is in a range of 0 / 100 to 10 / 90.

[0084] This means that the aluminium content, specified by the expression (1) described above, is set to 20 mol ppm, 200 mol ppm, 2000 mol ppm and 20000 mol ppm (2 mol-%) in the working examples E and 0 mol ppm, 10 mol ppm, 50000 mol ppm (5 mol-%) and 100000 mol ppm (10 mol-%) in the comparative examples 1.

[0085] Note that in the case where the aluminium content is 0 mol ppm, an aqueous zinc nitrate solution is prepared without using the aluminium nitrate-9-hydrate and then dropped into the aqueous ammonium carbonate solution.

[0086] For the resulting basic zinc carbonate, mineral phases are identified using an X-ray diffractometer (D8 ADVANCE, manufactured by Bruker Co., Ltd.) and the crystallite size is measured using the Debye-Scherrer method.

[0087] In addition, a measurement of thermal reduction is performed using a TG-DTA device (TG / DTA 6300, manufactured by Hitachi High-Technologies Corporation), a carbon analysis is performed using a combustion method with an analyzer (LECO CS844), and an analysis of Zn and Na is performed using an optical ICP emission spectrometer (ICP-9000, manufactured by Shimadzu Corporation).

[0088] Based on the results of X-ray diffraction and component analysis, it is determined that a basic zinc carbonate with hydrozincite as the main component is obtained.

[0089] Note that in comparative example 1, where the Al content is 10 mol%, a heterogeneous phase identified as zinc aluminum carbonate hydroxide hydrate predominates.

[0090] Analysis of the filtrate reveals a precipitate yield of 99%. Furthermore, it is determined that the thermal reduction, resulting from decarbonization and dehydration, is complete at approximately 600°C. (Heat treatment)

[0091] The resulting basic zinc carbonate is placed in an alumina crucible and subjected to a heat treatment for decarbonization and dehydration at 360°C in atmospheric conditions. The temperature rise rate is set to 2°C / min, the residence time at 360°C is six hours, and cooling is natural. This yields a zinc oxide powder. While the heat treatment temperature is preferably 250°C or higher, more preferably 350°C to 420°C, it can be selected according to the required sintered body properties. The present invention allows for a thorough investigation of the heat treatment temperature to clarify its effect on the untamped and tapped density. (Production of shaped bodies or molded bodies)

[0092] The resulting zinc oxide powder is passed through a 0.6 mm sieve for simple grinding and is press-molded at a pressure of approximately 60 MPa into two bodies: one disc-shaped body measuring 20 mm in diameter × 2 mm and the other a plate-shaped body measuring 40 × 40 × 5 mm. Each of the formed bodies is produced in batches of n = 15.

[0093] At this stage, considering the clear effects of differences in the powder characteristics of the zinc oxide powder resulting from the synthesis conditions on the molded and sintered bodies, granulation etc. using a spray dryer or similar equipment is not carried out. However, this does not apply to the production of actual products.

[0094] As will be described later, disc-shaped specimens are used as samples for evaluation using a scanning electron microscope (SEM), density measurement and X-ray diffraction, and plate-shaped specimens are used as samples for measuring electrical resistance and flexural strength. (Production of sintered bodies)

[0095] The manufactured disc-shaped and plate-shaped bodies are baked in the atmosphere. The baking temperature is set to 900–1200°C (in 100°C intervals), the residence time at this temperature is six hours, the temperature rise rate is 4°C / min, and they are then left to cool in a cooling oven. This results in the production of disc-shaped and plate-shaped sintered bodies. <Bewertung 1>

[0096] Various evaluations are carried out using the obtained sintered bodies.

[0097] The disc-shaped sintered bodies are examined using SEM to measure the size of the sintered grain (unit: µm).

[0098] After the plate-shaped sintered bodies have been processed into 30 mm × 4 mm × 4 mm rods, the volume resistivity or the through-resistance (unit: Ω·cm) is measured using a four-clamp method and the flexural strength (unit: MPa) is measured in accordance with ISO178.

[0099] The size of the sintered grain, the flexural strength, and the volumetric resistivity are set as average values ​​for 15 samples. The standard deviation and the coefficient of variation (= (standard deviation / average value) × 100) of the size of the sintered grain and the flexural strength are also determined. The coefficient of variation (unit: %) is a measure or index of the variation. The results are given in Table 1, Table 2, and Table 3 below.

[0100] Note that the sintered bodies were manufactured in the same manner as in Working Examples E and Comparative Examples 1, using a zinc oxide powder from the later described Reference Examples 2 (Synthesis Example 2) to measure the bulk resistivity. The results are also given below in Table 3. (Table 1) Baking temperature [°C] Characteristics of a sintered body Comparative example 1 Working example E Comparative example 1 1-1 1-2 E-1 E-2 E-3 E-4 1-3 1-4 Al content [mol ppm] 0 10 20 200 2000 20000 50000 100000 900 Size of desintered grain [µm] 1,10 1,00 0,80 0,70 0,60 0,55 0,50 0,50 Standard deviation 0,3890 0,3250 0,2480 0,1995 0,1500 0,1265 0,1100 0,1050 Coefficient of variation [%] 35,4 32,5 31,0 28,5 25,0 23,0 22,0 21,0 1000 size of the 2,70 2,20 1,80 1,40 1,00 0,70 0,65 0,65 sintered grain [µm] Standard deviation 0,8521 0,5940 0,4410 0,3080 0,2000 0,1260 0,1105 0,1073 Coefficient of variation [%] 31,6 27,0 24,5 22,0 20,0 18,0 17,0 16,5 1100 Size of desintered grain [µm] 3,69 3,80 3,60 2,60 1,90 1,70 1,70 1,60 Standard deviation 0,8900 0,5600 0,7200 0,4680 0,3230 0,2720 0,2550 0,2240 Coefficient of variation [%] 24,1 22,5 20,0 18,0 17,0 16,0 15,0 14,0 1200 Size of desintered grain [µm] 6,00 5,80 5,70 5,40 5,20 5,00 4,90 4,90 Standard deviation 1,5421 0,5600 1,1685 0,9450 0,8580 0,7500 0,6615 0,5880 Coefficient of variation [%] 25,7 22,0 20,5 17,5 16,5 15,0 13,5 12,0

[0101] As shown in Table 1 above, it is observed that working examples E-1 to E-4, where the Al content is 20 mol ppm or higher and 20,000 mol ppm or less (2 mol% or less), have smaller sintered grain sizes (many grain boundaries) than those in comparison examples 1-1 and 1-2, where the Al content is less than 20 mol ppm, even at any of the baking temperatures. There is a trend, however, that the lower the baking temperature, the smaller the sintered grain size.

[0102] The working examples E-1 to E-4, when the baking temperature is at least 900 °C and 1000 °C, have smaller coefficients of variation and less variation in the size of the sintered grain than those in the comparison examples 1-1 and 1-2. (Table 2) Baking temperature [°C] Characteristics of sintered body Comparative example 1 Working example E Comparative example 1 1-1 1-2 E-1 E-2 E-3 E-4 1-3 1-4 Al content [mol ppm] 0 10 20 200 2000 20000 50000 100000 900 Flexural strength [MPa] 107 109 111 109 105 100 95 90 Standard deviation 1,0486 1,0573 1,0545 1,0137 0,966 0,96 0,931 0,9 Coefficient of variation [%] 0,98 0,97 0,95 0,93 0,92 0,96 0,98 1 1000 Flexural strength [MPa] 121 125 125 126 123 116 110 100 Standard deviation 1,1253 1,15 1,1125 1,1088 1,0947 1,0556 1,023 0,95 Coefficient of variation [%] 0,93 0,92 0,89 0,88 0,89 0,91 0,93 0,95 1100 Flexural strength [MPa] 134 138 140 136 133 1285 122 114 Standard deviation 1,2328 1,2558 1,232 1,1832 1,1704 1,152 1,1224 1,0716 Coefficient of variation [%] 0,92 0,91 0,88 0,87 0,88 0,9 0,92 0,94 1200 Flexural strength [MPa] 140 144 145 142 137 132 126 120 Standard deviation 1,274 1,2816 1,2615 1,2212 1,1919 1,1616 1,1466 1,116 Coefficient of variation [%] 0,91 0,89 0,87 0,86 0,87 0,88 0,91 0,93

[0103] As shown in Table 2 above, working examples E-1 to E-4, in which the Al content is 20 mol ppm or higher and 20,000 mol ppm or less (2 mol% or less), exhibit higher flexural strengths than those in comparison examples 1-3 and 1-4, in which the Al content exceeds 20,000 mol ppm, even at any of the baking temperatures. The low flexural strength of comparison examples 1-3 and 1-4 is assumed to be caused by expansion resulting from the formation of a spinel phase (ZnAl₂O₄), which is a heterogeneous phase. (Table 3) Baking temperature [°C] Characteristics of sintered body Comparative example 1 Working example E Comparative example 1 Reference example 2 1-1 1-2 E-1 E-2 E-3 E-4 1-3 1-4 2-1 2-2 Al content [mol ppm] 0 10 20 200 2000 20000 50000 100000 200 20000 900 Volume resistivity [Ω·cm] 50000000 100000 14000 650 2500 40000 800000 5000000 7000 320000 1000 1500000 4500 100 200 500 10000 100000 600000 800 25000 1100 65000 400 60 35 40 80 800 6000 150 1450 1200 4000 25 4,2 2,6 3 7 50 300 50 300

[0104] As shown in Table 3 above, the working examples E-1 to E-4 show tendencies towards lower volume resistivity and more excellent conductivity than those in the comparison examples 1-1 to 1-4.

[0105] In particular, when the baking temperature is 900 to 10000°C, it is found that the working examples E-1 to E-4 have a lower volume resistivity, which decreases by about 2 places, than that in the comparison example 1-1 in which Al is not added. <Synthesebeispiel 2 (Referenzbeispiele 2)>

[0106] The zinc oxide powder without the addition of aluminum, prepared in Synthesis Example 1 (zinc oxide powder from Comparative Example 1-1), is added to aqueous aluminum nitrate solutions, mixed, and dried at 200°C, yielding zinc oxide powders with Al contents of 200 mol ppm and 20,000 mol ppm (2 mol%). It is assumed that the resulting zinc oxide powder contains precipitated aluminum, which is fixed as an amorphous hydroxide on the powder particle surfaces. <Synthesebeispiel 4 (Referenzbeispiele 4)>

[0107] A zinc oxide powder is produced in the following manner according to patent document 1.

[0108] Zinc nitrate 6-hydrate (manufactured by Kishida Chemical Co., Ltd.) is used as the zinc salt, aluminum nitrate 9-hydrate (manufactured by Kishida Chemical Co., Ltd.) is used as the aluminum salt, ammonium carbonate (manufactured by Kishida Chemical Co., Ltd.) is used as the carbonate, and 30 wt% sodium hydroxide (manufactured by Kishida Chemical Co., Ltd.) is used as the alkali.

[0109] Everything is the same as in Synthesis Example 1 up to an alkali precipitate synthesis procedure. A combination of zinc nitrate and aluminum nitrate, weighed out to a total of 0.5 mol, is dissolved in 1 L of pure water to prepare a mixed aqueous solution of zinc nitrate and aluminum nitrate.

[0110] 0.5 L of an aqueous 0.4 mol ammonium carbonate solution is prepared in a 2 L beaker.

[0111] The mixed aqueous solution of zinc nitrate and aluminum nitrate is added dropwise to a sodium bicarbonate solution at a rate of 1 L / h, while the sodium bicarbonate solution is stirred using a rotator at a rotation speed of approximately 50 rpm. Solution addition and pH regulation are carried out in the same manner as in Synthesis Example 1, and then 30 wt% sodium hydroxide is added dropwise to the sodium bicarbonate solution. This maintains a constant pH of 7.5 in the sodium bicarbonate solution while the mixed aqueous solution of zinc nitrate and aluminum nitrate is added dropwise. As a result, a precipitate is formed by the precipitate-generating reaction.

[0112] After the solution feed is complete, the solution is stirred and hardened for approximately 10 minutes using a rotator set to the same rotational speed of approximately 50 rpm as during the precipitation reaction. It is then immediately separated into solid and liquid, rinsed, and vacuum-dried, yielding a dried powder of basic zinc carbonate. A cooling device is used to maintain the temperature of the aqueous ammonium carbonate solution below 30°C throughout the precipitation reaction, stirring, and hardening.

[0113] Based on the results of the same analysis as in Synthesis Example 1, it is determined that a basic zinc carbonate with hydrozincite as a major component is obtained. Furthermore, the analysis of the filtrate reveals a precipitate yield of nearly 99%.

[0114] Using the obtained basic zinc carbonate, a heat treatment is carried out in the same manner as in Synthesis Example 1, yielding a zinc oxide powder. In Synthesis Example 4, the Al content is set to 0 mol ppm, 200 mol ppm, and 20,000 mol ppm (2 mol%). <Bewertung 2: Bewertung von Zinkoxidpulver>

[0115] The zinc oxide powders from Synthesis Example 1 (Comparative Examples 1 and Working Examples E) and Synthesis Example 4 (Reference Examples 4) are subjected to X-ray diffraction analysis using an X-ray diffractometer (D8 ADVANCE, manufactured by Bruker Co., Ltd.) to determine the crystallite size. They are then subjected to a specific surface area measurement using a BET extraction method with a BET-specific surface area measuring device (AUTOSORB-MP1, manufactured by Quantachrome Instruments) to determine the BET diameter. Furthermore, the untamped density and tapped density are determined using the methods described above. The results are given in Table 4 below. (Table 4) Al content [molppm] Crystallite size [nm] BET diameter [nm] Untamped density [g / cm³] 3 ] Tapped density [g / cm³] 3 ] Synthesis example 1 Comparative example 1 1-1 0 80 88,0 0,82 1,35 Working example E E-2 200 68 72,8 0,79 1,29 E-4 20000 52 54,6 0,68 1,12 Synthesis example 4 Reference example 4 4-1 0 85 93,5 0,36 0,59 4-2 200 50 53,5 0,24 0,39 4-3 20000 40 42,0 0,23 0,38

[0116] As shown in Table 4, the zinc oxide powders from Synthesis Example 1 exhibit a higher untamped density and a higher tapped density than those in Synthesis Example 4. Therefore, it can be expected that the zinc oxide powders from Synthesis Example 1 will be able to produce a dense sintered body even at a low temperature (e.g., 1000°C or less) when obtaining a shaped sintered body, since shrinkage is low due to the high filling density and the increase in contact points among the grains.

[0117] Furthermore, in Synthesis Example 1 and Synthesis Example 4, the temperatures (heat treatment temperatures) during the heat treatment of basic zinc carbonate are varied from exactly 360°C to within a range of 350°C to 420°C, and zinc oxide powders are produced to determine crystallite sizes and tapped densities. The results are shown in the graph of Fig. 3 indicated.

[0118] The Fig. Figure 3 is a graph showing the relationships between the tapped density and crystallite size of the zinc oxide powders of synthesis examples 1 and 4.

[0119] In the graph of Fig. Synthesis Example 1 (Working Examples E), in which the Al content is 20 mol ppm to 2 mol%, is represented by plots of white circles; Synthesis Example 1, in which Al is not added, is represented by plots of black circles; Synthesis Example 4, in which the Al content is 20 mol ppm to 2 mol%, is represented by white diamond-shaped plots; and Synthesis Example 4, in which Al is not added, is represented by black diamond-shaped plots. The plots of the respective synthesis examples include differences in the heat treatment temperature.

[0120] It is based on the graph of Fig. 3. It was found that if the crystallite size in synthesis example 1 is the same as in synthesis example 4, approximately twice the tapped density is obtained. There is hardly any discernible influence of the Al content on the crystallite size and tapped density. Synthesis example 2 shows no differences compared to synthesis example 1.

[0121] Here, the zinc oxide powders (Al is not added to either of them) from synthesis example 1 and synthesis example 4 are examined at an accelerating voltage of 3kV using a very low accelerating SEM.

[0122] The Fig. Figure 1 is a SEM image of a zinc oxide powder from synthesis example 1. Fig. Figure 2 is a SEM image of a zinc oxide powder from synthesis example 4. In synthesis example 1 ( Fig. 1) It is confirmed that the aggregation and coupling of particles that shape the zinc oxide powder are more significant than those in synthesis example 4 ( Fig. 2), thereby controlling excessive grain growth.

[0123] In even greater detail, by reducing the stirring force and shortening the stirring and hardening time during the precipitate-generating process and during stirring and hardening or post-treatment, the aim is to facilitate the formation of primary particles that are bound together to form flakes (e.g., see the one described later). Fig. 2), which are characteristic of layered hydroxide. In contrast, if the stirring and curing time is longer, primary particles repeatedly collide with each other as a result of the stirring, resulting in the loss of flake shapes, and therefore grain-like shapes (e.g., see the one described later) are expected. Fig. 1) be easily obtained.

[0124] While the reasons why the tapped density of the zinc oxide powder in synthesis example 1 is higher than that in synthesis example 4 (see Fig. 3), are unclear, the aggregation and coupling of particles that insignificantly configure the zinc oxide powder (see Fig. 1), and secondary particles formed by moderate aggregation, may be factors of this. <Bewertung 3: Bewertung von Formkörpern>

[0125] The zinc oxide powder from synthesis example 4 is press-molded in the same way as in synthesis example 1 to produce disc-shaped bodies with a diameter of 20 mm × 2 mm (n = 15).

[0126] Density (unit: g / cm³) 3 ) the disc-shaped molded body is determined.

[0127] The mold density is set to the average value for 15 samples, and the standard deviation and coefficient of variation (= (standard deviation / average) × 100) are determined. The coefficient of variation (unit: %) is a measure of the variation. The results are given in Table 5 below. (Table 5) Al content [molppm] Density of the molded body [g / cm³] 3 ] Standard deviation Coefficient of variation [%] Synthesis example 1 Comparative example 1 1-1 0 2,89 0,0228 0,79 Working example E E-2 200 2,77 0,0225 0,81 E-4 20000 2,48 0,0198 0,80 Synthesis example 4 Reference example 4 4-1 0 2,45 0,0294 1,20 4-2 200 2,40 0,0266 1,11 4-3 20000 2,38 0,0259 1,09

[0128] As shown in Table 5, when the molded parts of working examples E have the same Al content as those in reference examples 4, low variation and high density are observed. Therefore, the zinc oxide powders of working examples E are suitable for compression molding. <Bewertung 4: Bewertung von Sinterkörpern>

[0129] The zinc oxide powders of synthesis examples 2 and 4 are also press-molded and baked as in synthesis example 1, yielding disc-shaped sintered bodies (n = 15) and plate-shaped sintered bodies (n = 15).

[0130] The disc-shaped sintered bodies are subjected to a density measurement (unit: g / cm³). 3 ) and size of the sintered grain (unit: µm) were examined using SEM.

[0131] After the plate-shaped sintered bodies have been processed into bars of 30 mm × 4 mm × 4 mm, the flexural strength (unit: MPa) is measured in accordance with ISO178.

[0132] The sintered body density, sintered grain size, and flexural strength are set as the respective average values ​​for 15 samples. The standard deviation and the coefficient of variation (= (standard deviation / average value) × 100) are also determined for all sintered bodies. The coefficient of variation (unit: %) is a measure of the variation. The results are given in Tables 6, 7, and 8 below. (Table 6) Al content [mol ppm] Baking temperature [°C] Baking temperature [°C] Baking temperature [°C] 900 1000 1200 Density of the sintered body [g / cm³] 3 ] Standard deviation Coefficient of variation [%] Density of the sintered body [g / cm³] 3 ] Standard deviation Coefficient of variation [%] Density of the sintered body [g / cm³] 3 ] Standard deviation Coefficient of variation [%] Synthesis example 1 Comparative example 1 1-1 0 5,40 0,0486 0,90 5,45 0,0480 0,88 5,55 0,0477 0,86 Working example E E-2 200 5,35 0,0455 0,85 5,43 0,0461 0,85 5,58 0,0479 0,86 E-4 20000 5,30 0,0435 0,82 5,35 0,0433 0,81 5,45 0,0452 0,83 Synthesis example 4 Reference example 4 4-1 0 5,20 0,0598 1,15 5,28 0,0633 1,20 5,35 0,0653 1,22 4-2 200 5,15 0,0567 1,10 5,23 0,0653 1,25 5,30 0,0663 1,25 4-3 20000 5,10 0,0612 1,20 5,20 0,0598 1,15 5,25 0,0683 1,30 (Table 7) AlGehalt [mol ppm] Baking temperature [°C] Baking temperature [°C] Baking temperature [°C] 900 1000 1200 Flexural strength [MPa] Standard deviation Coefficient of variation [%] Flexural strength [MPa] Standard deviation Coefficient of variation [%] Flexural strength [MPa] Standard deviation Coefficient of variation [%] Synthesis example 1 Comparative example 1 1-1 0 107 1,0486 0,98 121 1,1253 0,93 140 1,2740 0,91 Working example E E-2 200 109 1,0137 0,93 126 1,1088 0,88 138 1,1868 0,86 E-4 20000 100 0,9600 0,96 116 1,0556 0,91 128 1,1264 0,88 Synthesis example 4 Reference example 4 4-1 0 95 1,1875 1,25 114 1,3680 1,20 125 1,4500 1,16 4-2 200 96 1,1808 1,23 110 1,2980 1,18 120 1,3560 1,13 4-3 20000 92 1,1592 1,26 104 1,2376 1,19 115 1,3225 1,15 (Table 8) Al content [molppm] Baking temperature [°C] Baking temperature [°C] 1000 1200 Size of sintered grain [µm] Standard deviation Coefficient of variation [%] Size of sintered grain [µm] Standard deviation Coefficient of variation [%] Synthesis example 1 Comparative example 1 1-1 0 2,70 0,8521 31,6 6,00 1,5421 25,7 Working example E E-2 200 1,40 0,3080 22,0 5,40 0,9450 17,5 E-4 20000 0,70 0,1260 18,0 5,00 0,7500 15,0 Synthesis example 4 Reference example 4 4-1 0 3,20 1,1264 35,2 7,20 2,2320 31,0 4-2 200 1,60 0,4480 28,0 6,00 1,3800 23,0 4-3 20000 0,80 0,1920 24,0 5,40 1,1880 22,0

[0133] As shown in Table 6, when the molded bodies of the working examples E have the same Al content as those in the reference examples 4, a low variation and high density can be observed.

[0134] Furthermore, as shown in Table 7 and Table 8, when the molded bodies of the working examples E have the same Al content as those in the reference examples 4, tendencies towards a small size of the sintered grain, a small variation thereof and a high strength can be observed.

[0135] In light of the above and considering the results shown in Table 3, the zinc oxide powders of the working examples E are suitable as a zinc oxide powder for the production of the zinc oxide sintered bodies, which enables the provision of a dense sintered body with excellent conductivity and high strength. 2. Zinc oxide varistor

[0136] Details of the zinc oxide varistor according to the embodiment of the present invention are described below. (1) Manufacturing process for zinc oxide varistor

[0137] The Fig. Figure 4 is a flowchart that depicts in chronological order a manufacturing process of a zinc oxide varistor (laminated varistor) according to the embodiment of the present invention, the focus being on the zinc oxide base material itself and the zinc oxide powder, which consists of the aforementioned particle size, tapped density, etc., is used.

[0138] First, the base material for a zinc oxide varistor is prepared. For this purpose, the synthesis (1) of a precursor in step S11 of Fig. 4. More precisely, using zinc nitrate hexahydrate, ammonium carbonate, and sodium hydroxide, as described above, and adjusting the pH of the ammonium carbonate to 7.5, a basic zinc carbonate suspension, which becomes a precursor, is prepared by a liquid-phase process. In the following step S13, the synthesis (2) of a precursor is carried out. That is, the basic zinc carbonate suspension obtained in step S11 is subjected to solid-liquid separation by suction filtration. Subsequently, the resulting excess sodium is rinsed and removed, and the solids after rinsing are vacuum-dried at 30°C for 20 hours to produce a dried powder from the basic zinc carbonate, which forms the precursor.

[0139] In step S15, the basic zinc carbonate obtained in step S13 is placed in an aluminum crucible and heat-treated for six hours in an atmospheric environment at 360 °C. Zinc oxide (ZnO) that meets the aforementioned crystallite size, grain diameter, untamped density, tapped density, etc., is obtained by the process of steps S11 to S15.

[0140] In step S17, the base material for a zinc oxide varistor is prepared and weighed. Here, either 0.5 mol% of an oxide of bismuth (Bi) or praseodymium (Pr), or 0.5 mol% of an oxide containing both Bi and Pr, is added to 100 mol% zinc oxide obtained in the previous steps. 0.5 mol% of one or more oxides of cobalt (Co), manganese (Mn), and nickel (Ni), which are transition metals, is then added. Furthermore, either 20,000 ppm or 20 ppm of one or more oxides of boron (B), gallium (Ga), and aluminum (Al), which are donor elements, are added to this composition. Donor elements reduce the resistance of the zinc oxide in the zinc oxide varistor and contribute to improved impulse strength, etc. It should be noted that other forms of the aforementioned additives besides oxides may be added.

[0141] In step S19, the varistor base material, weighed as described above, is crushed and measured in a ball mill, etc., and in the following step S21, a plasticizer, a dispersant, a diluent, etc., are added to produce a slurry. Then, in step S23, the slurry produced in step S21 is formed into a film using a doctor blade, for example, to produce a green film of approximately 10 to 100 µm.

[0142] In step S25, a capacitor pattern is printed with an electrode paste made of, for example, platinum (Pt), palladium (Pd), Ag / Pd, etc., and a multilayer body with the green film, on which an inner electrode is formed, is thermocompressed and laminated using a hot press or similar device. In the subsequent step S27, the laminated green film is cut to a predetermined product size and then diced.

[0143] In step S29, the laminated body is held at 500°C for ten hours after cube formation, for example, and the binder is removed. Then, in step S31, baking is carried out, for example, at 900°C. When observing a cross-section of the sintered body produced by such low-temperature sintering with a scanning electron microscope (SEM), the zinc oxide grains do not grow abnormally, their size and shape are uniform, and a dense structure with few voids between the zinc oxide grains is confirmed. As a result, the sintered zinc oxide body exhibits high flexural strength (high tensile strength) and high electrical conductivity.

[0144] In step S33, the aforementioned sintered body is annealed, for example, at 700°C. Then, in step S35, a terminal electrode (external electrode) is formed with Ag paste or Ag / Pd paste and baked at a predetermined temperature. It should be noted that the R-formation (chamfering) of the grains can be carried out in a centrifugal cylinder with an abrasive, etc., after the annealing process.

[0145] In step S37, the outer electrode formed in step S35 above is plated by electroplating, e.g., Ni layer, Sn layer. Next, in step S39, electrical properties such as varistor voltage and overvoltage current (impulse resistance) are investigated, during which the zinc oxide varistor is completed. (2) Evaluation of the zinc oxide varistor

[0146] The Fig. 5 and Fig. Figure 6 presents a diagram showing the evaluation results of the zinc oxide varistor produced by the procedures described above. For performance evaluation, bulk samples with an element size of 9.8 mm × 5 mm × 1.0 mm and electrode dimensions of 7.5 mm × 3.4 mm are used. Since varistor voltage and overvoltage current are correlated, the following are used here, as in [reference to relevant section]. Fig. 5 and Fig. Figure 6 shows a comparison of the zinc oxide varistor according to the embodiment and conventional products 1 to 3, with the horizontal axis representing varistor voltages and the vertical axis representing overvoltage currents and clamping voltage. Conventional product 1 is a zinc oxide varistor made with JIS powder, produced by the French method. Conventional product 2 is a zinc oxide varistor using a powder of dried basic zinc carbonate synthesized in Comparative Example 4 (precursor synthesis Example 5), and conventional product 3 is a zinc oxide varistor using a powder of dried basic zinc carbonate synthesized in Comparative Example 1 (precursor synthesis Example 2).

[0147] From the in the Fig. The evaluation results shown in sections 5 and 6 demonstrate that the properties of the zinc oxide varistor according to the embodiment (represented by a dashed line in the diagram where the linear approximation is applied to • and ▪) exhibit high surge current capability and low clamping voltages, and even at each of the varistor voltages, it displays excellent surge current capability and lower clamping voltage than conventional products 1 to 3. Furthermore, even with the same dimensions of the zinc oxide varistor according to the embodiment, the maximum surge current capability can be three times or greater than that of conventional products. This means that even miniaturizing the element to one-third of its size can achieve the same performance as conventional products.

[0148] On the other hand, it is noted that the aluminum content influences the varistor voltage of the zinc oxide varistor, as in the Fig. 5 and Fig. Figure 6 is given. That is to say, regarding the zinc oxide varistor using zinc oxide powder to which aluminum (Al) is added as a donor element, working example 1 yields the result of adding 20 ppm aluminum oxide, and working example 2 yields the result of adding 20,000 ppm aluminum oxide; based on these findings, it is determined that the more aluminum oxide is added, the higher the varistor voltage can be set. The results regarding the amount of aluminum added, the baking temperature, the varistor voltage, the clamping voltage, and the overvoltage withstand capability of the zinc oxide varistor are given in detail in Table 9. (Table 9) Baking temperature Characteristics Al content [mol ppm] [°C] 0 10 20 200 2000 20000 50000 100000 900 Size of desintered grain [µm] 1,10 1,00 0,80 0,70 0,60 0,55 0,50 0,50 Varistor voltage [V] 122 196 295 501 699 791 999 1412 Terminal voltage [V] 159 245 339 576 804 910 1149 1765 Overvoltage resistance [A] 1050 1670 3948 3095 2220 1560 620 335 1000 Size of desintered grain [µm] 2,70 2,20 1,80 1,40 1,00 0,70 0,65 0,65 Varistor voltage [V] 35,3 56,6 85,3 144,8 202,0 228,6 288,7 408,0 Clamp 45,8 70,8 98,0 166,5 232,3 262,9 332,0 510,1 voltage [V] Overvoltage resistance [A] 1343 2136 5050 4503 3230 2270 902 487 1100 Size of desintered grain [µm] 3,69 3,65 3,40 2,60 1,90 1,70 1,65 1,60 Varistor voltage [V] 14,2 15,3 34,3 39,2 81,2 86,9 109,8 155,1 Terminal voltage [V] 18,4 28,5 39,4 45,1 93,4 99,9 126,2 193,9 Overvoltage resistance [A] 2130 3387 8008 7138 5120 3598 1430 773 1200 Size of desintered grain [µm] 6,00 5,80 5,70 5,40 5,20 5,00 4,90 4,90 Varistor voltage [V] 9,0 9,2 10,0 10,2 11,0 12,0 12,7 13,1 Terminal voltage [V] 13,3 13,0 12,5 12,8 13,8 15,0 18,9 29,1 Overvoltage resistance [A] 428 681 1610 1455 1320 928 369 199

[0149] With regard to the zinc oxide varistor according to the embodiment of the present invention, zinc oxide is used with a crystallite size of 20 to 100 nm, a particle diameter of 20 to 110 nm, which is determined using a Specific Surface Area BET method, and an untamped density of 0.60 g / cm³. 3 or higher and a tapped density of 0.80 g / cm³ 3 or higher than a varistor material. Since there is no abnormal grain growth in the zinc oxide sintered body and there are some gaps between particles, the sintered particles have a uniform size and high density, resulting in a zinc oxide varistor with high overvoltage strength and low clamping voltage, while ensuring high strength and high electrical conductivity.

[0150] Furthermore, a dense zinc oxide sintered body can be obtained by lowering the sintering temperature, i.e., by low-temperature sintering at 900°C or less, due to the heat shrinkage behavior during sintering. This reduces the use of rare metals such as Pd, Au, and Pt, and thus the cost of the varistor. Additionally, the varistor can be miniaturized, as it exhibits three or more times the shock resistance of conventional products while maintaining the same performance.

[0151] Furthermore, the addition of aluminum (Al) as a donor element to the zinc oxide (ZnO) powder used for the zinc oxide varistor, such that the mol% of Al to zinc (Zn) is between 5 and 100,000 ppm, allows for the regulation of the sintered grain size in accordance with the amount of aluminum added. Such aluminum-enriched zinc oxide allows for the regulation of the sintered grain size and its reduction and variation by increasing the baking temperature (e.g., to 1200°C or less), even during compaction. For example, by setting the amount of aluminum added to 200 ppm for a baking temperature of 950°C or lower, and to 20,000 ppm for a baking temperature of 1050°C or higher, grain growth can be controlled and particle size regulated.

[0152] Note that one or more types of antimony oxide (Sb) and chromium (Cr) can be added to the base material mixture for the zinc oxide varistor of the embodiment to suppress and control grain growth. Furthermore, the silica composition (SiO2 system) can be added as a glass component to stabilize the sintering process.< / arbeitsbeispiele>

Claims

[1] Zinc oxide varistor comprising zinc oxide (ZnO) as a major component, one or more types of additives selected as a grain boundary component from a group consisting of bismuth (Bi) and praseodymium (Pr), and one or more types of additives selected as a transition metal element from a group consisting of cobalt (Co), manganese (Mn), and nickel (Ni); wherein the zinc oxide has a crystallite size of 20 to 100 nm, determined by X-ray diffraction, a grain diameter of 20 to 110 nm, determined by a BET method, and an untamped density of 0.60 g / cm³ 3 or higher, and a tapped density of 0.80 g / cm³ 3 or higher. [2] Zinc oxide varistor according to claim 1, wherein an amount of aluminium (Al) of 20 ppm to 20,000 ppm in a molar ratio is added to zinc (Zn) as a donor element. [3] Zinc oxide varistor according to claim 2, wherein the aluminum-doped zinc oxide produced by adding the aluminum (Al) is obtained by heat-treating a carbonate hydrate produced by precipitate-generating reactions in an aqueous aluminum salt solution, an aqueous zinc salt solution, an aqueous carbonate solution and an aqueous alkali solution. [4] Zinc oxide varistor according to claim 3, wherein the carbonate hydrate contains either a basic zinc carbonate, represented by the following expression (1), or a hydrate of basic zinc carbonate; M 4-6 (CO3) 1-3 (OH) 6-7 · nH2O (1) wherein M Zn 1-x Al x Let x be a number of 2×10 -5 denotes up to 0.02 and n denotes a number from 0 to 2. [5] Zinc oxide varistor according to claim 2, wherein either powder of the aluminum-doped zinc oxide is formed as is, or either after pulverization by means of a bead mill or granulation by means of a spray dryer is formed and sintered at a temperature of 1200°C or lower to obtain a zinc oxide sintered body. [6] Zinc oxide varistor according to claim 1 or 2, wherein one or more types of donor elements consisting of boron (B) and gallium (Ga) are further added.

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