A nickel sulfide inclusion, (also abbreviated to NiS), occurs during the process of manufacturing float glass (normal window glass).
The process of tempering float glass can cause a NiS to change from its normal state (known as a low-temperature structure) to a different high-temperature, crystalline structure. When the glass is cooled quickly (as part of the process) the NiS particle is unable to change back to its original form. Over a certain period of time NiS will slowly convert to the original low temperature structure. This means the NiS increases in size, and the mechanical stresses caused by this cause the tempered pane to shatter, for no apparent reason (hence spontaneous glass breakage).
US Patent Issued to Intel on June 21 for "Methods of Forming Nickel Sulfide Film on a Semiconductor Device" (Oregon Inventors)
Jun 22, 2011; ALEXANDRIA, Va., June 22 -- United States Patent no. 7,964,490, issued on June 21, was assigned to Intel Corp. (Santa Clara,...