Patent Number: 046844805
Section: description

The object of the invention is explained in greater detail in the examples that follow. As ceramic starting powders, the following powder types have been used in the examples. Boron carbide powder (A) contained 77.3% by weight B and 0.3% by weight B.sub.2 O.sub.3 and had a particle size distribution of 100% finer than 100 .mu.m PA1 90% finer than 60 .mu.m PA1 70% finer than 40 .mu.m PA1 50% finer than 30 .mu.m PA1 30% finer than 20 .mu.m and PA1 20% finer than 15 .mu.m PA1 100% finer than 50 .mu.m PA1 90% finer than 15 .mu.m PA1 70% finer than 10 .mu.m PA1 50% finer than 8 .mu.m PA1 30% finer than 6 .mu.m and PA1 20% finer than 4 .mu.m. PA1 100% finer than 50 .mu.m PA1 90% finer than 15 .mu.m PA1 70% finer than 10 .mu.m PA1 50% finer than 8 .mu.m PA1 30% finer than 5 .mu.m and PA1 20% finer than 2 .mu.m PA1 28 parts by weight boron carbide powder (A), PA1 65 parts by weight silicon carbide powder (C), PA1 7 parts by weight graphite powder, PA1 18 parts by weight phenolic resin powder and PA1 6 parts by weight furfural PA1 60 parts by weight boron carbide powder (B) PA1 20 parts by weight silicon carbide powder (C) PA1 10 parts by weight graphite powder PA1 19 parts by weight phenolic resin powder and PA1 6 parts by weight furfural. PA1 93 parts by weight boron carbide powder (B) PA1 7 parts by weight graphite powder PA1 19 parts by weight pheonolic resin powder and PA1 5 parts by weight furfural. Boron carbide powder (B) contained 78.8% by weight B, 0.24% by weight B.sub.2 O.sub.3 and had a particular size distribution of Silicon carbide powder (C) contained 98.4% by weight SiC and had a particle size distribution of The graphite powder was a screened natural graphite fraction of 40 .mu.m and finer. The lexural strength was determined according to the 3-point method with samples which measured 45.times.4.5.times.3.5 mm, a span of 30 mm amd a rate of load application of 1.8 N/mm.sup.2 per second. For the corrosion test, the plates were immersed for 2000 or 3000 hours in water near its boiling point, and the flexural strength was again determined. The percent decrease in the flexural strength refers to the initially measured value at room temperature. EXAMPLE 1 were homogenously mixed. The powder mixture was pressed under a pressure of 28 MPa into plates 2 mm thick. The plates were stacked between glass plates and cured by heating for 15 hours at 180.degree. C. The plates were then stacked between graphite plates and heated to 1050.degree. C. in the absence of air, the time for heating to 1050.degree. C. was 19 hours and the plates were held at this temperature for 7 hours. Properties of the plates produced: ______________________________________ density 2.0 g/cm.sup.3 boron content: 19.0% by weight corresponds to 19.5% by volume B.sub.4 C silicon content: 36.5% by weight corresponds to 32.5% by volume SiC free carbon content: 20.0% by weight corresponds to 18.0% by volume free C B.sup.10 loading: near 0.014 g Boron 10/cm.sup.2 flexural strength: 35 N/mm.sup.2 compression strength: 60 N/mm.sup.2 modulus of elasticity: 22,000 N/mm.sup.2 radiation resistance: 10.sup.11 rad (no measurable change in the flexural strength and the dimension) flexural strength after immersion for 2000 hours in water at 93.degree. C.: 25 N/mm.sup.2 flexural strength after immersion for 3000 hours in water at 93.degree. C.: 24 N/mm.sup.2 corresponds to a drop of 31.4% compared to the initial value. ______________________________________ EXAMPLE 2 Under the same conditions as described in Example 1, the following homogeneous powder mixture was prepared and compressed and the plates cured and fired in the absence of air: Properties of the plates produced (2mm): ______________________________________ density 1.85 g/cm.sup.3 boron content: 41.0% by weight corresponds to 39.0% by volume B.sub.4 C silicon content: 17.5% by weight corresponds to 14.5% by volume SiC free carbon content: 19.0% by weight corresponds to 16.0% by volume free C B.sup.10 loading: near 0.028 g Boron 10/cm.sup.2 flexural strength: 25 N/mm.sup.2 compression strength: 50 N/mm.sup.2 modulus of elasticity: 16.000 N/mm.sup.2 radiation resistance: 10.sup.11 rad (no measurable change the flexural strength and the dimensions) flexural strength after immersion for 2000 hours in water at 93.degree. C.: 17.0 N/mm.sup.2 flexural strength after immersion for 3000 hours in water at 93.degree. C.: 16.0 N/mm.sup.2 corresponds to a drop of 36% compared to the initial value. ______________________________________ EXAMPLE 3 (for comparison) Under the same conditions as described in Example 1, the following homogeneous powder mixture was prepared and compressed and the plate cured and fired in the absence of air. Properties of the plates produced (2 mm): ______________________________________ density: 1.70 g/cm.sup.3 boron content: 64.0% by weight corresponds to 55% by volume B.sub.4 C free carbon content: 16.0% by weight corresponds to 12% by volume free carbon B.sup.10 loading: 0.04 g Boron 10/cm.sup.2 flexural strength 16 N/mm.sup.2 compression strength: 50 N/mm.sup.2 modulus of elasticity: 11.500 N/mm.sup.2 radiation resistance: 10.sup.11 rad (no measurable change in flexural strength and the dimensions) flexural strength after immersion for 2000 hours in water at 93.degree. C. 8.5 N/mm.sup.2 flexural strength after immersion for 3000 hours in water at 93.degree. C.: 7.5 N/mm.sup.2 corresponds to a drop of 52.3% compared to the initial value. ______________________________________