Grain boundary diffusion is a processing technique that applies heavy rare earth elements like dysprosium or terbium only at the grain boundaries of a sintered NdFeB magnet, rather than throughout the bulk alloy. It raises coercivity and thermal stability while using far less of the expensive heavy rare earth material than bulk alloying would require. In this dataset it shows up both as a manufacturing process claim and, in newer filings such as US11921174B1, as the basis for diagnostic methods that measure whether the diffusion has caused irreversible demagnetization.

H01F covers magnets, inductors and transformers, and it was one of the three IPC classes used to build this search gate alongside C22C38 and B22F3. Because the search itself was anchored on magnet-specific terms like coercivity and sintered magnet, nearly every qualifying record naturally falls into H01F. The more informative comparison is what sits underneath it — B22F powder metallurgy and C22C alloy processing each cover under half the corpus, which is where distinct technical approaches are actually being differentiated.

Patent publication typically lags the actual filing date by around eighteen months, so the most recent year in any filing trend is structurally undercounted. Companies like Shin-Etsu Chemical, TDK and Xiamen Tungsten showing zero filings in the latest tracked year most likely reflects that lag rather than a genuine halt in R&D. It is worth revisiting the same assignees' filing counts a year or two after this data cut-off before concluding they have exited the space.

Not on the evidence here. Filings ran at 110 in 2017, peaked at 121 in 2020, and had already fallen to 109 by the 2022 midpoint before the recent-year figures dropped further under the publication lag. That trajectory looks like a filing cycle that has already peaked rather than one still building, though the composition layer around coercivity and heavy rare earth reduction remains heavily claimed.

The thinner IPC classes in this dataset point to it: C04B ceramics-adjacent processing, C23C and B05D coating routes for corrosion protection, and H02K motor-integration claims all carry far fewer records than the core H01F, B22F and C22C classes. Diagnostic and measurement methods layered on top of established manufacturing processes, similar in spirit to the grain boundary diffusion demagnetization test in US11921174B1, also sit in less crowded claim territory than the underlying alloy and sintering processes themselves.