JPS6031795B2 - Zirconia sintered body - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a zirconia fired body, and more particularly to a zirconia cryogenic body having high thermal shock strength.

A pure zirconia sintered body transforms from a monoclinic crystal structure to a monoclinic crystal structure around 1100 oo, and has a Sariko 24
It transforms into a cubic crystal structure around 00oo.

On the other hand, during the cooling process, a transformation opposite to the above occurs, but especially when transforming from a tetragonal system to a monoclinic system, a large volumetric expansion is accompanied, so in order to prevent destruction due to this volumetric expansion, First, oxides such as ittria, magnesia, and lucidia are precipitated onto zirconia to obtain a stabilized zirconia sintered body consisting of zirconia with a cubic crystal structure (hereinafter referred to as cubic zirconia). There is. However, since cubic zirconia has a high tensile modulus on the hot side, stabilized zirconia compacts have a drawback of low thermal shock strength. On the other hand, a partially stabilized zirconia sintered body, in which cubic zirconia and zirconia with a monoclinic crystal structure (hereinafter referred to as monoclinic zirconia) coexist, has a tetragonal crystal structure. When the crystal structure of zirconia (hereinafter referred to as tetragonal zirconia) transforms into monoclinic zirconia, the microcracks that occur around the monoclinic zirconia absorb the fracture energy caused by thermal shock, resulting in stabilization. It is said to have higher thermal shock strength than zirconia baked stripes.

However, the degree of improvement was not so remarkable because it still contained cubic zirconia with a large thermal extensibility coefficient. On the other hand, 82
A zirconia aggregate in which ~97 (mol %) of tetragonal zirconia and 18-3 (mol %) of monoclinic zirconia coexist, and in which zittria is solidified, is produced by tension, compression,
There are reports that it has high mechanical strength in bending and shearing, as well as improved hardness and toughness.

However, this competitive body had a drawback of low thermal shock strength because the proportion of monoclinic zirconia was extremely small at 18 to 3 (mol %). In other words, a small proportion of monoclinic zirconia means that the amount of transformation from tetragonal zirconia to monoclinic zirconia is small. Since the thermal shock energy is mainly absorbed by the microcracks generated in the thermal shock energy, depending on the amount of microcracks generated due to the small amount of transformation, sufficient absorption of thermal shock energy is not achieved. An object of the present invention is to solve the above-mentioned drawbacks of conventional zirconia sintered bodies and to provide a zirconia sintered body having extremely high thermal shock strength.

To achieve the above object, the present invention provides a sintered body made of zirconia having a substantially monoclinic crystal structure, wherein the loose solid body in parentheses contains 6 to 11 (mol%) of magnesia as a solid solution. It is characterized by zirconia Akatsuki crystals.

In the present invention, the term "a competitive body consisting of zirconia with a substantially monoclinic crystal structure" refers to a zirconia fired body analyzed by X-ray diffraction within a diffraction angle range of 20 to 40 (degrees). In the case of cubic zirconia (111) and (200), tetragonal zirconia (111), (002)
This means that the (200) diffraction pattern cannot be detected.

Next, the zirconia sintered body (hereinafter referred to as aggregate) of the present invention
will be explained in detail along with its manufacturing method.

First, extremely fine zirconia powder and magnesia powder having an average particle size of IA or less are prepared.

Next, the zirconia powder and magnesia powder are mixed so that the amount of magnesia powder is 6 to 11 (mol %) based on the whole.

Next, the above mixture is calcined at 800 to 1200 (00) for several hours, and then pulverized with a pole mill.

The raw material powder is obtained by repeating such calcining and pulverization. This raw material powder forms a solid port in which zirconia powder and magnesia powder are evenly mixed together, and the crystal structure of the solid solution is usually monoclinic. Next, the raw material powder is molded into a desired shape by a well-known molding method such as a rubber press method, an injection molding method, a mold molding method, or an extrusion molding method,
Obtain a molded body. Next, the above-mentioned molded body was placed in a heating furnace, and 1
After the temperature is gradually raised to 600 to 1800 (00), the temperature is maintained for several hours to several tens of hours for firing.

In the process of temperature increase, the crystal structure of the solid solution changes as follows:
Transforms from a monoclinic system to a tetragonal system, a cubic system, or a combination of both. The temperature and speed of such crystal structure transformation vary depending on the purity of the zirconia and magnesia powders used and the amount of magnesia powder mixed.

Therefore, with reference to the phase diagram, the firing temperature at which the crystal structure described above is obtained is determined. This firing temperature is 1600 to 1800 (00) as described above. Next, the fired body is slowly cooled to about 100 mm at a rate of 200 to 2100 degrees Celsius/hour, and further cooled in a furnace to room temperature.The action of magnesia during this cooling process will be explained below. The crystal structure of the fired body is a cubic system or a state in which a tetragonal system and a cubic system coexist.

And the amount of magnesia is 6 to 11 (mol%) as described above.
If the range is within the range of A gradual transformation of the crystal structure into a monoclinic, tetragonal, and cubic coexistence state occurs, and the microcracks generated by this transformation are uniformly dispersed. Microcracks themselves absorb the destructive energy. Therefore, destruction of the fired body during the cooling process can be prevented. On the other hand, if the amount of magnesia is less than the above-mentioned lower limit, that is, less than 6 mol%,
The fired body has a crystal structure that is either tetragonal or a combination of tetragonal and cubic systems, but because the amount of magnesia is too small, the tetragonal system changes to monoclinic system as it cools. A sudden transformation of the crystal structure occurs, microcracks occur throughout the fired body, and the fired body is destroyed by the energy.

In addition, if the amount of magnesia exceeds the above-mentioned upper limit, that is, 11 mol%, the fired product has a cubic crystal structure or a crystal structure in which a tetragonal system and a cubic system coexist; In the process, the state changes from a cubic system or a coexistence state of a tetragonal system and a cubic system to a state of a coexistence of a monoclinic system and a tetragonal system, a state of a coexistence of a monoclinic system and a monoclinic system, or a state of a coexistence of a monoclinic system and a monoclinic system. The transformation to a state in which the crystal system, tetragonal system, and cubic system coexist does not proceed easily, and if an attempt is made to cause the above transformation by making the cooling time extremely long, the crystals grow and the crystal grains become large. Next, the fired body cooled to room temperature was heated to a temperature of 1300 to 145
After gradually raising the temperature to 0 (° C.), the material is kept at that temperature for several hours to several tens of hours for aging.

In this process, the crystal structure of the fired body transforms into a tetragonal system or a coexistence state of a monoclinic system and a tetragonal system. The temperature and speed of this transformation vary depending on the crystal structure of the fired body before aging and the amount of solidified magnesia, so the aging temperature at which the crystal structure described above is determined is determined with reference to the phase diagram. The aging temperature is 1,300 to 1,450 mm as described above. Next, the fired body is gradually aged from the aging temperature to about 1000 qo at a slow rate of 5 to 100 qo (00/hour), and further cooled to room temperature to obtain the fired body of the present invention.

The action of magnesia in this cooling process will be explained below. The crystal structure of the fired body after aging is a tetragonal system or a coexistence state of a monoclinic system and a tetragonal system.

When magnesia is in the above range of 6 to 11 (mol%), the crystal structure described above gradually transforms into a monoclinic crystal structure as it cools, and the microorganisms generated by this transformation occur. Since the cracks are uniformly distributed, the microcracks themselves absorb the fracture energy caused by the generation of microcracks. Therefore, destruction of the fired body during the cooling process can be prevented. In addition, since the volumetric expansion accompanying the above transformation forms a compressive stress field within the fired body,
Energy due to compressive stress is stored within the crystalline structure, leading to improved mechanical strength. On the other hand, when the amount of magnesia is less than 6 mol%, the fired product has a crystal structure that is tetragonal or a monoclinic and tetragonal system coexisting. Because the amount of magnesia is too small, the crystal structure rapidly transforms from tetragonal to monoclinic as it cools, causing microcracks to occur throughout the fired product, and the resulting energy destroys the fired product. I end up. In addition, when the amount of magnesia exceeds 11 mol%, the fired product has a cubic crystal structure or a crystal structure in which a tetragonal system and a cubic system coexist. However, the transformation to monoclinic system does not progress easily, and cubic or tetragonal crystal structures may remain in the late crystals. If an attempt is made to force the above transformation by taking an extremely long cooling time, the crystals will grow and the crystal grains will become large.
In this way, by the above cooling, the Akatsuki body of the present invention consisting essentially of monoclinic zirconia is obtained, but in order to do so, it is necessary to adjust the amount of magnesia powder mixed in the mixture at the stage before obtaining the raw material powder to within the above range. That is, 6 to 11 (mol%
) (of course, this range does not change even if it becomes a sintered body).

and the cooling rate after aging from 5 to 100 (℃/hour)
It is necessary to do so. The presence of monoclinic zirconia and magnesia dramatically improves the thermal shock strength of the fired product.In other words, the zirconia that makes up the fired product has a monoclinic crystal structure. This means that a sufficient amount of microcracks are present near or around the monoclinic zirconia.

Therefore, when a crack occurs in the dawn compact due to thermal shock, the propagation of the crack is obstructed by the micro-cracks, making it difficult to propagate as it follows a winding course, thereby improving the thermal shock strength. In addition, when the bran feeder is rapidly heated, distortion occurs in the Akatsuki due to thermal expansion, but as the heating progresses, the crystal structure transforms from monoclinic to tetragonal, and at this time approximately There is a volumetric contraction of 3%, and this volumetric contraction acts to alleviate the above-mentioned strain, so that the thermal shock strength is improved. Magnesia, which is solid-solved in the competitive body, is caused by the transformation of the pseudo-chatite, which has been transformed into a tetragonal crystal structure under heat WS and force, and transforms back into a monoclinic crystal structure upon cooling. This prevents the burnt stripes from being destroyed by suppressing the speed of the burns. Magnesia also provides good oxygen ion conductivity to the Akatsuki compact. In other words, pure zirconia competitive striations have a monoclinic crystal structure, but since the oxygen ion density is small, in order to increase this, the competitive striations have to have a cubic crystal structure. Approximately 240 Akatsuki Keitai
It becomes necessary to heat to a high temperature of 0 qo or more. However, the firing body of the present invention can obtain a considerably large transport number even at about 1400. Therefore,
Coupled with its high thermal shock strength, it is very suitable as a constituent material of so-called solid electrolyte oxygen sensors, which measure the oxygen concentration in molten steel, for example. The competitive structure of the present invention has a crystal grain size of 10 to 100 (French), has a monoclinic crystal structure inside each crystal grain, and has an average grain size of 0.1 It is preferable that fine crystal grains (subgrains) having a particle size of 1 to 1 (mu) are uniformly dispersed, and that the proportion of the fine crystal grains is 2% by weight or more. That is, even if the average particle diameter of the fine crystal grains is less than 0.1 μm, even if it exceeds 1 μm, the thermal shock strength tends to decrease, and even if the content is less than 2% by weight, the same problem occurs. Either case is unfavorable because the thermal shock strength tends to decrease. Here, the average particle size is calculated as follows.

That is, first, the Akatsuki compact is cut, the cut surface is polished, and if necessary, chemically etched, and then a microscopic photograph of the surface is taken. Then, define a section of a certain area on this photo, and add the areas of particles existing in that section from large particles to small particles until the total area becomes 1/2 of the area of the above section. . Next, the average area obtained by dividing this added value by the number of particles from which the added value was obtained is assumed to be a circle, and the average particle diameter is determined. That is, a, ten a2 ten a3 ten... ten an=1
/2S2an 汀d2n 4 -・-d=2 no harm However, d: Average particle size an: Area of each particle (n〒1÷2,3,...) S: Area of section Also, within the crystal grain The weight content of a certain monoclinic microcrystalline grain can be determined by cutting a sintered body, optically polishing the cut surface, and then observing it with a scanning electron microscope. It can be measured by making a sample and observing it with a transmission electron microscope.

As mentioned above, the sintered body of the present invention has a high thermal shock strength, exhibits oxygen ion conductivity even at relatively low temperatures, and has relatively high mechanical strength, so it can be used in a variety of applications. It can be used for.

An example is shown below. A Solid electrolyte oxygen sensors such as metallurgical sensors and combustion control sensors for internal combustion engines, gas stoves, boilers, etc. B. For industrial machinery and equipment such as crucibles, various dies, tanditsch nozzles, protection tubes, bolts, nuts, various valves, mechanical seals, various parts such as nozzles and combustion chambers of coal and oil combustion equipment, dovets of internal combustion engines, etc. parts.

Hereinafter, the present invention will be explained in more detail based on Examples.

Example Using zirconia powder and magnesia powder with an average particle size of 0.1 mm, seven types of Zro2-Mgo shown in the table were prepared.
(In the table, the numbers written to the lower right of zirconia and magnesia are their amounts expressed in mol%).

That is, after mixing zirconia powder and magnesia powder in the amounts shown in the table, this was mixed at 1000 oo
The material was calcined for an hour, and then ground in a pot mill for two hours, and this calcining and grinding process was repeated twice to produce a raw material powder.

Next, a 2% polyvinyl alcohol aqueous solution was added as a binder to the raw material powder and mixed well, and after drying, a plate-shaped molded body was produced by a rubber press method. Next, the above molded body is fired and cooled under the conditions shown in the table to make a sintered body, and this sintered body is cut and polished to have a thickness of 3 sides, a width of 3 ribs, and a length of 2 mm.
Four rib samples were made.

Next, the thermal shock strength of each of the above samples was measured.

The results are shown in the table. Thermal shock strength was determined by dropping a plate-shaped sintered body into water heated to an arbitrary temperature of Tx°C and rapidly cooling it, and then measuring its bending strength by a well-known three-point bending test method.

Then, the heating temperature T at which the bending strength begins to decrease
x°C is read as the critical temperature Tc°C, and this critical temperature T
c. The difference Tc-T (〇0) between 0 and the temperature T°C of the water was used as an index.

Note that the measurement conditions for the three-point bending test method are a span length of 2 m and a load application acceleration of 1 skin/min. Table (*) Cool from the firing temperature to 1200°C at a rate of 100°C/hour, and then furnace cool to room temperature.

Claims (1)

[Claims] 1 A sintered body made of zirconia with a substantially monoclinic crystal structure, and this sintered body contains 6 magnesia.
A zirconia sintered body characterized by containing ~11 (mol%) in solid solution.