New silver powders with large surface area as heat exchanger materials
✍ Scribed by W. Itoh; A. Sawada; A. Shinozaki; Y. Inada
- Publisher
- Elsevier Science
- Year
- 1991
- Tongue
- English
- Weight
- 346 KB
- Volume
- 31
- Category
- Article
- ISSN
- 0011-2275
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✦ Synopsis
The properties of several silver powders made by chemical processing, such as surface area, electrical resistivity and the Kapitza resistance with liquid 3He, have been measured. The results indicate that the finest new powder (C-8), with a large specific surface area (6 m 2 g-l), is suitable for use as a heat exchanger material at ultralow temperatures.
areas of C-8 and G-1 are 6 and 0.3 m 2 g-t, respectively, before the sintering procedure. The differences in surface area of the C-8 powder among the batches used were within 5 %. A scanning electron microscope picture of the C-8 powder is shown in Figure 1. As can be seen, the particles are almost spherical. The diameters range from 500 to 1000 A.
After the presintering process, the powders were placed in a Teflon tube of inner diameter 6 mm and length 17 mm, and pressed using a brass cylinder and piston. The final length was 10 mm and the packing density was about 50% of that of bulk silver. The pressed material in the Teflon tube was put into a stainless steel oven. The oven was pumped for = 30 min and then filled with hydrogen gas at an absolute pressure of 1 bar t. The oven was then warmed to the desired temperature, which was measured by a thermocouple thermometer located close to the sample. It took ---45 min to reach the final temperature, which was maintained for the required length of time. The oven heater was then switched off and the oven allowed to cool to room temperature naturally. The flow of hydrogen gas was maintained throughout the process at a flow rate of 5 cm 3 min -~.