Note that as TNP was ramped to 2200 K, two discontinuities, each decreasing Fz by 1.3%, were observed in Fz (green trace). Discontinuities resulting from thermionic electron emission are common when NPs are first heated to high TNP, however, these increase Fz. Decreasing Fz implies that the discontinuities here resulted from decreasing Q, i.e., from thermal cation emission, presumably emission of some surface contaminant with low ionization energy. The silica NP feedstock was reported to have ≤ 0.002% by Ca and Al, for example, but it is also difficult to avoid contamination by environmental Na during handling. Cation emission from hot NPs is, therefore, not unexpected, and it was seen frequently for silica NPs as they were first heated. Curiously, however, cation emission from Si NPs and from graphite, graphene, carbon black, and diamond NPs, was quite rare, presumably reflecting differences in composition and work function.