7.  Intangible Assets

Intangible assets were fully amortized as of June 30, 2020 and March 31, 2020. Amortization expense for the intangible assets was zero and $0.1 million for the three months ended June 30, 2020 and 2019, respectively.

The manufacturing license provides the Company with the ability to manufacture recuperator cores previously purchased from Solar Turbines Incorporated (“Solar”). 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 $6,000 and $7,000 were earned by Solar for the three months ended June 30, 2020 and 2019, respectively. Earned royalties of approximately $59,000 and $52,000 were unpaid as of June 30, 2020 and March 31, 2020, respectively, and are included in accounts payable and accrued expenses in the accompanying condensed consolidated balance sheets.