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”).

Solar Turbines Incorporated

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 $5,600 and $6,000 were earned by Solar for the three months ended June 30, 2021 and 2020, respectively. Earned royalties of approximately $59,400 and $53,800 were unpaid as of June 30, 2021 and March 31, 2021, respectively, and are included in accounts payable and accrued expenses in the accompanying Condensed Consolidated Balance Sheets.

Amortization

As of June 30, 2021 and March 31, 2021 intangible assets related to Solar were fully amortized. Amortization expense of other intangible assets was approximately $34,000 for the three months ended June 30, 2021 and 2020.