7.  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 December 31, 2020 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 $7,700 and $6,200 were earned by Solar for the three months ended December 31, 2020 and 2019, respectively. Royalties of approximately $20,100 and $22,600 were earned by Solar for the nine months ended December 31, 2020 and 2019, respectively. Earned royalties of approximately $73,000 and $52,000 were unpaid as of December 31, 2020 and March 31, 2020, respectively, and are included in accounts payable and accrued expenses in the accompanying condensed consolidated balance sheets.

Amortization expense for the intangible assets was zero and $0.1 million for the three and nine months ended December 31, 2020, respectively. For the three and nine months ended December 31, 2019, amortization expense was $0.1 million and $0.2 million, respectively.