JPH07277834A - Composite ceramic - Google Patents

Abstract

PURPOSE:To obtain a composite ceramic excellent in mechanical strength and toughness and having low heat conductivity by forming a solid soln. having a specified compsn. contg. Si, Al, O and N. CONSTITUTION:A powdery mixture consisting of 20-50wt.% (expressed in terms of metallic silicon) Si, 20-49wt.% Al6Si2O13 (mullite) of 2MgO.2Al2O3.5SiO2 (cordierite) and the balance oxides of Y, La, Ce and Dy is compacted. The resultant compact is fired under a reaction in a nitrogen atmosphere at <=1,450 deg.C and it is heated to >=1,700 deg.C to obtain the objective composite ceramic as a compact contg. an SixAlyOzNw type solid soln. This ceramic has 5-15% porosity and <=5W/m.K heat conductivity and contains a phase of the solid soln. in which (x) is >4.69.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a composite ceramic mainly containing silicon, aluminum, oxygen and nitrogen which is excellent in mechanical strength, toughness and heat insulation (low thermal conductivity), and a method for producing the same.

[0002]

2. Description of the Related Art Composite ceramics are excellent in heat resistance, thermal shock resistance, wear resistance, and have high mechanical strength, so that they are used as wall materials and walls of combustion chambers exposed to high temperatures and pressures of heat engines and thermal equipment. Have been tried for practical use, and some have been put to practical use. However, in order to reduce the heat loss of the heat engine or the heat equipment, a ceramic having lower thermal conductivity is required.

Japanese Patent Application Laid-Open No. 56-13456 discloses a composite ceramics.
According to the technique disclosed in Japanese Patent Publication No. 8, silicon (Si) at 80 vo% or less and alumina (Al
₂ O ₃ ) and 0 to 15 vo% magnesium oxide (Mg
O) and powder are formed into a desired shape and the temperature is 120
After reaction firing at 0 to 1400 ° C, temperature 1500 to 1800
When heated at ℃, a sintered compact having a porosity of 5 to 15% containing a silicon nitride (Si3 N4) solid solution is obtained.

Further, in the technique disclosed in Japanese Patent Laid-Open No. 58-60676, silicon corresponding to 60 to 98.9 wt% of silicon nitride is added to chromium (Cr), chromium oxide (Cr ₂ O ₃ ) and nitriding. 0.1 to 15 wt% selected from chromium (CrN)
1 selected from oxides of rare earth elements, aluminum oxide, and zirconium oxide (ZrO ₂ )
.About.25 wt% of one or more kinds of materials are made into powder, mixed and molded, and the molded body is subjected to reaction sintering and further sintering in a non-oxidizing atmosphere.

However, all of the techniques are focused on enhancing the mechanical properties, that is, the toughness of the sintered compact formed of the composite ceramics, and are not intended to obtain the composite ceramics having a low thermal conductivity. It cannot be said that the thermal conductivity of each composite ceramic is extremely low.

[0006]

SUMMARY OF THE INVENTION In view of the above problems, an object of the present invention is to provide a composite ceramic which contains silicon, aluminum, oxygen and nitrogen as main raw materials and has excellent mechanical strength and toughness and low thermal conductivity. To do.

[0007]

In order to achieve the above object, the structure of the present invention has a porosity of 5 to 15% and Si
An x Aly Oz Nw type solid solution is formed, a phase in which the value of x in the solid solution exceeds 4.69 is formed, and the thermal conductivity is 5 W / m · K or less. is there.

[0008]

The present inventor has made the Si constituting the composite ceramics.
It has been found that even in the x Aly Oz Nw type solid solution, there is a region where the thermal conductivity is lowered due to the stoichiometric composition and the existing amount. In other words, silicon (Si) and mullite (Al ₆ Si
₂ O ₁₃ ) or cordierite (2MgO · 2Al ₂ O ₃
・ 5SiO ₂ ), yttrium (Y), lanthanum (L)
a), an oxide of a substance selected from cerium (Ce), and dysprosium (Dy) are mixed in a predetermined ratio to prepare a molded body, and after firing the molded body once,
When sintered at a temperature of 1550 ° C or higher, Six Aly Oz N
The value of x in the w-type solid solution exceeds 4.69, and a solid solution with large anharmonicity is formed, and the thermal conductivity becomes very low.

[0009]

EXAMPLES In the present invention, as a raw material for composite ceramics,
20 to 50 wt% of silicon (Si) converted to metallic silicon,
20～49Wt% mullite (Al ₆ Si ₂ O ₁₃₎ or cordierite _{(2MgO · 2Al 2 O 3 ·} 5SiO 2)
And the balance being an oxide of a substance selected from yttrium (Y), lanthanum (La), cerium (Ce), and dysprosium (Dy) [eg yttrium oxide (Y
₂ O ₃ )] is used to form a molded body from these mixed powders.

The molded body is reacted and fired in a nitrogen atmosphere at a temperature of 1450 ° C. or lower, and then heated to a temperature of 1700 ° C. or higher to obtain a sintered molded body made of a composite ceramic containing a solid solution of Six Aly Oz Nw type. The obtained sintered compact has a porosity of 5 to 15%, and Six Aly Oz Nw.
A solid solution of the type is formed, and the value of x in the solid solution is 4.
Over 69. The reason why the porosity and the value of x in the solid solution are limited is the value necessary for achieving both high strength and low thermal conductivity of the composite ceramics.

1 to 4 show silicon (Si) and mullite (Al).
₆ Si ₂ O ₁₃ ), yttrium oxide (Y ₂ O ₃ ), and β silicon nitride (β-Si ₃ N ₄ ) powders of each sintered body prepared by changing the compounding ratio and the reaction sintering temperature. The relationship between the characteristics and the production phase is shown. Figure 1 shows the firing temperature and (100) of α-silicon nitride.
The line 31 represents the relationship between the peak intensity of the solid solution and the peak intensity of the solid solution (peak intensity ratio), and the line 31 is Si ₁₈ A1 of silicon nitride containing 8.4 wt /% yttrium oxide.
_0.2 O _1.2 N _1.8 (110) plane, line 3
2 is for the (301) plane of Si ^4.69 A _11.31 O _1.31 N _6.69 , and line 33 is Si ₆ Al ₁₀ O ₂₁ N ₄ (2θ = 2
3 to 26 °).

FIG. 2 shows the relationship between the firing temperature and the porosity of the composite ceramic containing yttrium oxide at 8.4 wt /%. Line 41 in FIG. 3 represents the relationship between the firing temperature and the thermal conductivity of the composite ceramic containing yttrium oxide 2.0 wt /%, and the line 42 represents the firing temperature and the thermal conductivity of the composite ceramic containing yttrium oxide 8.4 wt /%. Represents the relationship. Figure 4
Represents the relationship between the firing temperature and the mechanical strength of the composite ceramic. FIG. 5 shows an X-ray diffraction pattern of the composite ceramic.

As is apparent from FIGS. 1 and 2, the firing temperature is
The higher the temperature, the more the sintered body becomes Si _1.8Al_0.2O_1.2
N_1.8Mold from Si₆Al_TenO_{twenty one}N_FourChange to type and S
i₆Al_TenO_{twenty one}N_FourThe mold content increases, and the porosity p
Is reduced. However, as shown in FIG. 3, the thermal conductivity λ is
The higher the firing temperature, the higher the firing temperature.
It can be seen that after showing a high price, it gradually decreases. Temperature 15
The increase in the thermal conductivity λ with the increase up to 50 ° C. is due to the porosity p
Due to the decrease in the temperature above the firing temperature above 1550 ° C
The decrease in thermal conductivity λ with increasing temperature causes large anharmonicity.
This is due to the formation of a solid solution.

As shown by the line 51 in FIG. 4, as the firing temperature increases, the four-point bending strength Kic increases, and as shown by the lines 52 and 53 in FIG. 4, yttrium oxide (Y
_The fracture toughness σ is improved as compared with the case where the amount of ₂ O ₃ ) is not added. However, the composite ceramics containing more than 15 wt% of yttrium oxide (Y ₂ O ₃ ) cracked when heated to a temperature of 1000 ° C. in the atmosphere. It is considered that the cause of this crack generation is due to the difference in (dispersed phase / matrix) of α-silicon nitride.

[0015]

As described above, the present invention provides a sintered composite ceramic having a porosity of 5 to 15%.
A phase in which the value of x exceeds 4.69 is formed in the Aly Oz Nw type solid solution, and the presence of the solid solution does not impair the mechanical strength and toughness and results in a low composite ceramic with low thermal conductivity. It comes at a cost.

Therefore, for example, if the composite ceramics of the present invention is applied to wall materials or structural members such as cylinders, cylinder heads and pistons that partition the combustion chamber of an internal combustion engine, the internal combustion engine can be cooled and insulated. The heat cycle efficiency can be improved.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a firing temperature and a peak intensity ratio of a composite ceramic according to the present invention.

FIG. 2 is a diagram showing a relationship between firing temperature and porosity of the composite ceramics.

FIG. 3 is a diagram showing the relationship between the firing temperature and the thermal conductivity of the composite ceramic.

FIG. 4 is a diagram showing the relationship between the firing temperature and the mechanical strength of the composite ceramic.

FIG. 5 is a diagram showing an X-ray diffraction pattern of the composite ceramics.

Claims (5)

[Claims] 1. A porosity of 5 to 15%, Six Aly
An Oz Nw type solid solution is formed, a phase in which the value of x exceeds 4.69 is formed, and the thermal conductivity is 5 W / m · K or less. 2. The Six Aly Oz Nw type solid solution is S
The composite ceramic according to claim 1, which is i ₆ Al ₁₀ O ₂₁ N ₄ . 3. The peak intensity reflected from a Six Aly Oz Nw type solid solution at the time of X-ray diffraction is (10) of α silicon nitride.
When the peak intensity reflected from the (0) plane is 1, x =
The peak intensity reflected from the (301) plane of the 4.69 solid solution and the value of 2θ of the solid solution of x = 6 are 23 to 26.
The composite ceramics according to claims 1 and 2, characterized in that the sum with the peak intensity reflected between ° C is greater than 1.75. 4. The composite ceramic according to claim 1, wherein at least one element selected from the group of elements yttrium (Y), lanthanum (La), cerium (Ce) and dysprosium (Dy). Then, the amount converted to the oxide weight of this is included in the range not exceeding 15 wt%,
The composite ceramic according to claim 1. 5. 20 to 50 wt% of silicon (Si) in terms of metallic silicon and 20 to 49 wt% of Al ₆ Si ₂ O ₁₃ (mullite) or 2MgO.2Al ₂ O ₃ .5SiO ₂ (cordierite). And the balance is yttrium (Y), lanthanum (La), cerium (Ce), dysprosium (Dy)
A molded body is molded from a mixed powder consisting of the oxide of the above, and the molded body is reacted and fired in a nitrogen atmosphere at a temperature of 1450 ° C. or lower, and then heated to a temperature of 1700 ° C. or higher to give Six Al.
A method for producing a composite ceramic, which comprises obtaining a molded product containing a y Oz Nw type solid solution.