Heat-resistant ceramics

Abstract

The objective is to provide heat-resistant ceramics and a method for manufacturing heat-resistant ceramics that have excellent thermal shock resistance without using petalite. [Solution] The heat-resistant ceramic is composed of iron oxide containing wustite, magnetite, and hematite, and a clay mixture containing Amakusa pottery stone or kaolinite, sericite, and quartz, which are the main crystalline phases contained in Amakusa pottery stone. Furthermore, the mixing ratio of the iron oxide to the Amakusa pottery stone, or the iron oxide to the clay mixture, contains at least 5% by mass of the iron oxide.

Description

[Technical Field]

[0001] The present invention relates to a ceramic excellent in thermal shock resistance used for heating cooking utensils such as household tableware and pots. [Background Art]

[0002] Heat-resistant ceramics are widely and generally used as materials used in environments where abrupt temperature changes occur, such as tableware and cooking utensils. As a heat-resistant clay raw material used for heat-resistant ceramics, one mainly composed of clay and a low-expansion material composed of fused quartz, petalite and the like is known (for example, Patent Document 1).

[0003] Petalite, which is a type of mineral, has a low coefficient of thermal expansion, and thus has an effect of suppressing cracking that occurs when heat-resistant ceramics are cooled. In addition, since petalite has a high melting point, it has the effect of improving heat resistance when firing heat-resistant ceramics, and is generally used in the manufacture of cooking utensils, tableware and the like that are resistant to temperature changes. [Prior Art Documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent No. 7276812 [Summary of the Invention] [Problems to be Solved by the Invention]

[0005] In recent years, demand for petalite has increased because it contains a large amount of lithium used in fuel cells of electric vehicles that are rapidly spreading, and a state of supply shortage has continued worldwide. Along with this, the trading price of petalite has soared, leading to the problem that it has become difficult to use petalite as a raw material for heat-resistant ceramics.

[0006] Therefore, an object of the present invention is to provide a heat-resistant ceramic excellent in thermal shock resistance without using petalite. [Means for Solving the Problems]

[0007] The raw materials for the heat-resistant ceramics of this disclosure consist of iron oxide containing wustite, magnetite, and hematite, and a blended clay containing Amakusa pottery stone or the main crystalline phases contained in Amakusa pottery stone: kaolinite, sericite, and quartz. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide heat-resistant ceramics with excellent thermal shock resistance and a method for manufacturing the same. [Brief explanation of the drawing]

[0009] [Figure 1] This figure shows an example of an X-ray diffraction (XRD) pattern obtained by different firing atmospheres in one embodiment of the present invention. [Modes for carrying out the invention]

[0010] A heat-resistant ceramic according to one embodiment of the present invention will be described. The heat-resistant ceramic is composed of powdered iron oxide consisting of particles having different crystalline structures of wustite (FeO), magnetite (Fe3O4), and hematite (Fe2O3), and Amakusa pottery stone.

[0011] Wustite is an imperfect iron oxide composed of iron and oxygen, possessing a cubic crystal structure (sodium chloride type structure) in which iron and oxygen atoms are regularly arranged. Wustite is mainly formed at high temperatures of approximately 570 degrees Celsius or higher in oxygen-deficient environments.

[0012] Magnetite, also known as iron oxide, is an iron oxide with a cubic crystal structure (spinel type structure) and possesses very strong magnetism.

[0013] Hematite, also known as red iron ore, is an iron oxide with a hexagonal crystal structure (corundum type). Unlike magnetite, which contains a mixture of divalent and trivalent iron, hematite is in a trivalent oxidation state and is therefore very stable. It also has high resistance to oxidation-reduction reactions, making it highly resistant to corrosion.

[0014] Amakusa pottery stone is a type of ore used as a raw material for ceramics, found in the Amakusa region of Kumamoto Prefecture. The main component of Amakusa pottery stone is kaolinite, and it also contains quartz, sericite, and other minerals. Furthermore, it contains trace amounts of feldspar (alkali feldspar and plagioclase), calcite, ilmenite (titanium oxide mineral), and montmorillonite.

[0015] Next, we will describe a method for manufacturing tableware made of heat-resistant ceramics. First, prepare a powder containing three crystalline structures of iron oxide: wustite, magnetite, and hematite, and powdered Amakusa pottery stone containing at least kaolinite, quartz, and sericite. The particle size of the iron oxide is preferably 0.5 to 50 μm, and the particle size of the Amakusa pottery stone is preferably 40 to 100 μm.

[0016] Once the above-mentioned powders are prepared, they are mixed (ST1: mixing step). Specifically, a predetermined amount of water is added to the powdered Amakusa pottery stone, and the mixture is stirred in a ball mill for a predetermined time to produce a clay-like mixture. At this time, the mixing ratio of Amakusa pottery stone to water is preferably about 7:3.

[0017] Next, a mixture (20% by mass) of powdered Amakusa pottery stone and water is stirred and mixed with powdered iron oxide (80% by mass) containing wustite, magnetite, and hematite until uniform. At this time, it is preferable to remove air bubbles by vacuum degassing. This produces a slurry containing powdered iron oxide, Amakusa pottery stone, and water, which is the raw material for heat-resistant ceramics.

[0018] The content ratio of iron oxide including wustite, magnetite and hematite relative to the entire slurry is desirably 0.1% by mass or more and less than 95% by mass. This is because when the content ratio of iron oxide exceeds 95% by mass, forming of the slurry becomes difficult.

[0019] In producing the slurry, iron oxide, Amakusa pottery stone and water may be stirred and mixed all at once; however, since iron oxide does not mix with water (strictly speaking, it is dispersed in water), uniformly dispersing iron oxide may become difficult if it is mixed into water from the beginning. Therefore, adding iron oxide after Amakusa pottery stone is sufficiently mixed with water makes it easier for iron oxide to be uniformly dispersed.

[0020] Next, the produced slurry is formed into a predetermined shape (ST2: forming step). Examples of the forming method include hand-building forming, in which the shape is adjusted by hand without using tools; wheel throwing forming, in which the shape is adjusted by hand or tools while rotating a potter's wheel with the slurry placed thereon; and pressure forming, in which the shape is adjusted by placing the slurry in a mold and applying mechanical pressure.

[0021] Next, the formed slurry is dried over 1 to 2 days, and a fixed amount of moisture added during forming is removed to stabilize the shape (ST3: drying step). Thereby, a so-called green body is completed.

[0022] Next, the green body is bisque fired (ST4: bisque firing step). In this step, the green body is hardened by firing at 600 to 900°C. Through this step, water absorbency increases, making it easier for a glaze, which will be described later, to uniformly adhere to the body.

[0023] Next, glazing is performed on the bisque-fired sample (green body) (ST5: glazing step). This step is performed by, for example, an operator applying the glaze with a brush, spraying it with a sprayer, or immersing the sample in a liquid glaze. Applying the glaze to the surface of the sample produces gloss and changes the texture and color. The glaze has a composition containing iron oxide having at least one crystal phase selected from wustite, magnetite and hematite. The glaze may contain all of wustite, magnetite and hematite.

[0024] Next, oxidative firing (main firing) is performed (ST6: firing step). Specifically, the molded and dried sample (green body) is fired at a temperature of 1200°C to 1300°C for 15 hours while supplying a predetermined amount of oxygen into the kiln. By supplying sufficient oxygen into the kiln, the molded body can be placed in an environment where oxidation easily proceeds, and oxides in the molded body are maintained in a stable oxidized state. Oxidative firing densifies the fired ceramic, and the glaze melts to turn the surface vitreous, thereby improving thermal shock resistance. The surface of the heat-resistant ceramic after oxidative firing may be polished as a final finishing process. Thereby, tableware (heat-resistant ceramic) is completed.

[0025] As described above, the raw material of the heat-resistant ceramic according to the present embodiment includes iron oxide containing wustite, magnetite and hematite, and Amakusa pottery stone. As a result of analyzing the fired heat-resistant ceramic by XRD diffraction (X-ray diffraction), it was confirmed that quartz, mullite and hematite were formed as crystal phases. It is considered that the physical properties of hematite changed through high-temperature treatment to form a more stable trivalent crystal structure. As described above, in the heat-resistant ceramic after firing, firing of iron oxide and Amakusa pottery stone produces at least quartz, hematite and mullite. The body formed by firing iron oxide and Amakusa pottery stone only needs to contain at least one of quartz, hematite and mullite.

[0026] Furthermore, the method for manufacturing heat-resistant ceramics in this embodiment includes a first step of generating a slurry by mixing powdered iron oxide containing wustite, magnetite, and hematite with a mixture of powdered Amakusa pottery stone and water, and a second step of firing the molded body produced in the first step at a temperature of 1200 degrees Celsius or higher and 1300 degrees Celsius or lower.

[0027] When the heat-resistant ceramics of the present invention, manufactured through the steps (ST1) to (ST6) described above, were subjected to a heat resistance test (thermal shock strength), they withstood a temperature difference (350 degrees Celsius) without cracking. In other words, the heat-resistant ceramics of the present invention demonstrated excellent thermal shock resistance without the use of petalite.

[0028] Furthermore, the applicant prepared heat-resistant ceramics with the following patterns (a) to (g) for the mixing ratio of iron oxide and Amakusa pottery stone before firing, and conducted thermal shock tests after firing. (a) 10% iron oxide, 90% Amakusa pottery stone (b) 20% iron oxide, 80% Amakusa pottery stone (c) 30% iron oxide, 70% Amakusa pottery stone (d) 40% iron oxide, 60% Amakusa pottery stone (e) 50% iron oxide, 50% Amakusa pottery stone (f) 70% iron oxide, 30% Amakusa pottery stone (g) Iron oxide 80%, Amakusa pottery stone 20%

[0029] The experimental results confirmed that all patterns (a) to (g) withstood a thermal shock of a temperature difference of 350 degrees. Furthermore, it was confirmed that heat-resistant ceramics with a composition ratio of 95% iron oxide and 5% Amakusa pottery stone had poor moldability and a reduced yield (a decrease in the production rate of heat-resistant ceramics that met a certain quality). In other words, if the composition ratio of iron oxide and Amakusa pottery stone before firing contains at least 5% by mass of iron oxide, it has the potential to withstand a thermal shock of a temperature difference of 350 degrees.

[0030] In this specification, "heat-resistant ceramics (ceramics)" refers to ceramics that have undergone oxidation firing. Furthermore, "slurry" refers to a mixture of Amakusa pottery stone and water that has been stirred and mixed with powdered iron oxide containing wustite, magnetite, and hematite, in its pre-molding state.

[0031] The present invention is not limited to the embodiments described herein and can be designed and modified without departing from the spirit of the invention. For example, the iron oxide contained in the heat-resistant ceramic may include substances other than wustite, hematite, and magnetite. Alternatively, instead of Amakusa pottery stone, a blended clay (an artificial clay mixture) containing kaolinite, sericite, and quartz, which are the main crystalline phases contained in Amakusa pottery stone, may be used. In such cases, in the mixing step (ST1), a mixture of the aforementioned powdered blended clay and water is used instead of powdered Amakusa pottery stone. It should be noted that if the iron oxide content of the heat-resistant ceramic and the blended clay before firing is at least 5% by mass, it has the potential to withstand a thermal shock of a temperature difference of 350 degrees. Furthermore, even in heat-resistant ceramics manufactured using blended clay, the material produced by firing the iron oxide and blended clay (the body) contains at least one of quartz, hematite, or mullite.

[0032] Next, with reference to Figure 1, other examples of the firing process (ST6) will be described. Figure 1 shows an example of X-ray diffraction (XRD) patterns obtained for a sample (body portion) of the heat-resistant ceramic of the present invention under different firing atmospheres (oxidizing atmosphere, neutral atmosphere, reducing atmosphere) in the kiln. The experiment was conducted on the final finished product after it had cooled to room temperature (room temperature state). The experimental results shown in Figure 1 are an example of an evaluation conducted to confirm the effects of the present invention, and the present invention is not limited to these specific conditions or results.

[0033] The samples were molded, dried, and bisque-fired according to (ST1) to (ST5) described above, and then fired according to (ST6). Three firing atmospheres were set for step (ST6): (A) oxidizing atmosphere, (B) neutral atmosphere, and (C) reducing atmosphere. The firing temperature was set to 1220°C and the firing time to 15 hours.

[0034] A neutral atmosphere is one that is more reducing than an oxidizing atmosphere, but not as reducing as a reducing atmosphere. For example, in a neutral atmosphere, the CO concentration can be set to a range of 0.5% to 1.5% at a firing temperature of 1220°C. Conversely, a reducing atmosphere can be set to a CO concentration of 3% or higher.

[0035] X-ray diffraction measurements were performed on each sample after firing under the three conditions described above, and diffraction patterns were obtained in the range of 2θ = 4° to 70°. In the sample fired in an oxidizing atmosphere, diffraction peaks mainly thought to originate from hematite were observed, and the peak corresponding to magnetite located around 2θ = 35° was virtually not confirmed.

[0036] Next, in the sample fired in a neutral atmosphere, a diffraction peak originating from magnetite was observed around 2θ=35°. Similarly, in the sample fired in a reducing atmosphere, a diffraction peak originating from magnetite was also observed around 2θ=35°, and the relative intensity of this diffraction peak tended to be equivalent to or greater than that in the neutral atmosphere. Thus, it was confirmed that a magnetic magnetite crystalline phase is formed in the material after firing in a neutral or reducing atmosphere. Furthermore, a diffraction peak originating from hematite was also observed in the sample fired in a neutral atmosphere. This suggests that the sample is in a multiphase state, mainly consisting of a magnetite phase but also containing a hematite phase.

[0037] Samples fired in a neutral or reducing atmosphere, i.e., finished heat-resistant ceramics, exhibited the behavior of attracting a permanent magnet when brought near, confirming that magnetism was imparted to the clay body. Magnetic materials are known to exhibit hysteresis loss and eddy current loss associated with changes in magnetization under alternating magnetic fields. Therefore, heat-resistant ceramics containing a magnetite crystalline phase in the clay body can respond to the alternating magnetic field formed by electromagnetic induction cookers (so-called IH cookers).

[0038] Conventional ceramic cookware for use with induction cooktops required extra steps, such as laying a conductive sheet on the bottom of the pot. However, by forming a magnetite crystalline phase in the material itself, as in the present invention, such extra steps can be eliminated, and the heat-resistant ceramic itself may react to the magnetic field of the induction cooktop and generate heat.

[0039] Thus, the firing process of (ST6) may be not only firing under an oxidizing atmosphere, but also firing under a neutral atmosphere or a reducing atmosphere. Furthermore, the clay produced by firing iron oxide containing wustite (FeO), magnetite (Fe3O4), and hematite (Fe2O3) with Amakusa pottery stone, or with said iron oxide and mixed clay, under a neutral or reducing atmosphere will contain at least magnetite. Moreover, when fired under any of the oxidizing, neutral, or reducing atmospheres, the clay after firing will contain at least one of wustite, magnetite, and hematite.

[0040] Furthermore, the glaze layer formed on the surface of the clay after firing contains, in its composition, either a crystalline phase of magnetite or iron oxides to precipitate magnetite during the firing process. As a result, not only does the clay itself generate heat under an alternating magnetic field, but the outermost glaze layer itself can also directly respond to induction heating. With this configuration, the heat generated from the glaze layer, which is close to the heat source, is efficiently transferred to the object being heated, significantly improving the thermal responsiveness of the cooking utensil. [Industrial applicability]

[0041] This material provides heat-resistant ceramics with excellent thermal shock resistance without the use of petalite, making it applicable to a wide range of uses, including tableware, cookware, and industrial heat-resistant materials.

Claims

1. Heat-resistant ceramics composed of iron oxide containing wustite, magnetite, and hematite, and a clay mixture containing Amakusa pottery stone or the main crystalline phases contained in Amakusa pottery stone: kaolinite, sericite, and quartz.

2. The heat-resistant ceramic according to claim 1, wherein the mixing ratio of the iron oxide and the Amakusa pottery stone, or the mixing ratio of the iron oxide and the clay mixture, before firing contains at least 5% by mass of the iron oxide.

3. The heat-resistant ceramic according to claim 1 or 2, wherein the body produced by firing the iron oxide and the Amakusa pottery stone, or the iron oxide and the blended clay, contains at least one of quartz, hematite, or mullite.

4. The surface is glazed, The heat-resistant ceramic according to claim 1 or 2, wherein the glaze is composed of at least one of wustite, magnetite, and hematite.

5. The heat-resistant ceramic according to claim 1, wherein the body produced by firing the iron oxide and the Amakusa pottery stone, or the iron oxide and the blended clay, in a neutral or reducing atmosphere contains magnetite.

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