Plug electrodes conventionally have used nickel and platinum. These were not pure metals, but alloys containing other metals. Depending on the composition of the alloy, the matrix material's properties can be maximized. Using the same approach, iridium is alloyed with rhodium. The rhodium layer covers the iridium, improving the properties of the iridium. This iridium/rhodium technology was recognized by the Patent Offices of Japan and the UK, and in January 1999, a patent was granted in each respective country. (Patent Number - Japan: 2877035, UK: 2302367). Because DENSO had this patented technology, DENSO was able to make dia. of the iridium down to 0.4mm.
The most important things for plug performance are ignitability and sparkability. To make the electrode as fine as possible is the goal of spark plug manufacturers worldwide, and DENSO has taken the lead, succeeding the commercializing the 0.4mm dia. center electrode plug.