| 摘要 |
<p>1459252 Resistors DANFOSS AS 28 Feb 1974 [1 March 1973] 9033/74 Heading HIS [Also in Division H5] A silicon carbide resistor body, e.g. for use as a heater, containing free silicon in its terminal portions only, is made from a member composed throughout of silicon carbide containing free silicon by subjecting the portion between the terminal portions to an etching process to remove the free silicon. Preferably the portion which is etched is thinner, and the terminal portions masked during etching in a HF-HNO 3 mixture. In one method silicon from a melt rises upwards by capillary action in a porous block consisting of SiC granules and colloidal graphite with which the silicon reacts to form SiC. If silicon is added to the melt silicon monoxide vapour is produced and reacts with the graphite to give a highly porous surface layer in which the silicon concentrates. The reaction, performed e.g. at 1500‹ C. in vacuo is terminated while the free silicon is still concentrated at the base of the body and hence in the terminal portions if the body is of appropriate form e.g. is U-shaped or consists (Fig. 3) of a tube with longitudinal slots extending from one end to spiral slots 12, 13 defining a resistance path of double spiral form. The average concentration of free silicon in the terminal portions may be 2-20% by volume and the surface concentration up to 50%. Contacts of silver, aluminium or gold-antimony alloy are conventionally deposited and then heated to form a eutectic alloy with the silicon.</p> |
