5. Intangible Assets

Intangible assets, net of amortization include intellectual property such as manufacturing licenses providing the Company with the ability to manufacture recuperator cores previously purchased from Solar Turbines Incorporated (“Solar”) and were fully amortized as of March 31, 2021 and March 31, 2020. The Company is required to pay a per-unit royalty fee over a seventeen-year period for cores manufactured and sold by the Company using the technology. Royalties of approximately $27,400 and $26,400 were earned by Solar for Fiscal 2021 and 2020, respectively. Earned royalties of approximately $53,800 and $52,500 were unpaid as of March 31, 2021 and 2020, respectively, and are included in accrued expenses in the accompanying consolidated balance sheets.

Amortization expense for the intangible assets was $0.1 million and $0.2 million for Fiscal 2021 and 2020, respectively.