JPH026999B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention is an improvement on a heat exchanger made of silicon carbide. Since the heat exchanger heats the low-temperature fluid by using the heat held by the high-temperature fluid through the partition walls, materials with good heat conductivity are used for the members forming the partition walls. In this case, copper and copper alloys are often used for low temperatures, but heat-resistant metals such as Inconel and Hastelloy are used for temperatures below 1000°C. Furthermore, at high temperatures exceeding 1000°C, the corrosion resistance of metal materials is extremely reduced, making them almost unusable. For this reason, it is possible to use ceramic materials such as carbon, silicon carbide, and cordierite, but ceramics have poor workability and reliability, and complex shapes cannot be obtained. For example, silicon carbide is essentially an oxidation-resistant material, but conventional clay-bonded silicon carbide is particularly susceptible to softening under load at high temperatures, and the formed SiO ₂ film changes in quality, resulting in SiO With the formation of _{No. 2} , the difference in stress between the inside and the surface gradually increased, and eventually the coating peeled off, making it impossible to fully demonstrate its properties. In order to make full use of the characteristics of silicon carbide, the present invention has investigated its shortcomings and improved them to develop an excellent heat exchanger that has never been seen before. ,Be,
Sintered silicon carbide is made using sintering aids such as Mg and Ti. That is, in the oxidation of silicon carbide, silicon carbide itself is oxidized, a SiO ₂ film is formed on its surface, and this SiO ₂
When the coating peels off, the newly exposed silicon carbide undergoes oxidation. In this case, if the generated SiO ₂ film exists without peeling off,
Oxidation of silicon carbide stops, but this SiO ₂ film
It has become clear that the presence of impurities such as Fe ₂ O ₃ and Na ₂ O transforms the composition into a thermally unstable composition, leading to peeling of the SiO ₂ film. In addition, this phenomenon causes oxidation to diffuse not only to the surface of the molded product but also to the inside through the pores, so the state of the pores also has a great effect on the oxidation resistance. Therefore, in the present invention, by making the silicon carbide molded body itself dense, oxidation is limited to only the surface area, thereby extending the life of the product.
In order to make it dense, when sintering silicon carbide powder,
0.05 to 5.50% by weight of boron is added in terms of B, and in order to further improve the thermal conductivity of the fired crystal, it is added to Al, Be, Mg, Ti in terms of metals.
At least one component is added in an amount of 0.05 to 5.50% by weight. That is, silicon carbide powder is self-sintered by molding and firing silicon carbide powder to which 0.05 to 5.50% by weight of each of at least one of Al, Be, Mg, and Ti is added as a sintering aid. This results in a dense compact with very few grain boundaries, so there is virtually no part with thermally or chemically inferior properties to silicon carbide, and the oxidation reaction is ultimately limited to the surface layer of the compact. That will happen. This contributes to extending the life of the molded body and results in a sintered body having high thermal conductivity. In the present invention, B is added to serve as an auxiliary agent for sintering submicron SiC powder, and if the proportion is outside this range, a dense sintered body that is gas-impermeable cannot be obtained. In addition, adding at least one component among Al, Be, Mg, and Ti, which may be a metal or an oxide, in terms of metal improves the thermal conductivity of the silicon carbide molded body self-sintered with B. If it is less than 0.05% by weight, almost no effect will be observed, and if it is more than 5.50% by weight, it will not only adversely affect the sinterability and make it difficult to form a dense product, but also reduce the grain boundary area of the sintered body. These additives present in the silicon carbide particles are more susceptible to corrosion than the silicon carbide particles, and as a result, it is difficult to form a corrosion-resistant sintered body. Examples of the present invention will be described below. 2.5% by weight of B and 2.5% by weight of Be were blended into α-SiC powder with an average particle size of 0.5μ, mixed and molded with 2% by weight of phenol resin as a primary binder, and this was sintered at 2050°C. By tying the bulk density
It had physical properties of 3.13g/cc and thermal conductivity of 200w/mk. This shows a thermal conductivity approximately four times higher than that of a conventional sintered body without the addition of Be. This sample was heated to 1250°C in air to measure its lifespan until cracks occurred due to oxidation, and it was 88 days. For comparison, samples were prepared in which only the addition ratio of Be was changed, and their physical properties are shown in the table below.

[Table] In the table, lifespan (days) is the number of days until cracks are visually observed. Comparative tests were also conducted on Al, Mg, and Ti instead of Be, and almost the same results were obtained. The effects that can be expected from the present invention can be applied not only to double-tube type engines, but also to multi-tube types, or heat storage type, heat transfer type, etc. of gas turbine engines for automobiles. Furthermore, in the case of the sintered body of the present invention, which is further coated with a silicon carbide film by CVD (chemical vapor deposition), the coating is formed densely and firmly, which further improves oxidation resistance and extends the lifespan. can be extended.

Claims (1)

[Claims] 1. In a heat exchanger that heats a low-temperature fluid with a high-temperature fluid through a partition wall, at least a portion of the partition wall contains silicon carbide powder with a boron component of 0.05 in terms of B. ~5.50% by weight and in terms of metal
At least one component among Al, Be, Mg, and Ti is 0.05%
A silicon carbide heat exchanger characterized by using a sintered silicon carbide material containing ~5.50% by weight. 2. The silicon carbide heat exchanger according to claim 1, wherein at least a part of the surface is coated with a silicon carbide film formed by a CVD method.