7.  Intangible Assets

The Company recorded amortization expense of $0.2 million and $0.3 million for the three and six months ended September 30, 2013, respectively. Intangible assets consisted of the following (in thousands):

The Company recorded amortization expense of $0.1 million and $0.3 million for the three and six months ended September 30, 2012, respectively. Intangible assets consisted of the following (in thousands):

Expected future amortization expense of intangible assets as of September 30, 2013 is as follows:

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 $22,100 and $16,600 were earned by Solar for the three months ended September 30, 2013 and 2012, respectively. Royalties of approximately $47,300 and $32,600 were earned by Solar for the six months ended September 30, 2013 and 2012, respectively. Earned royalties of approximately $22,100 and $24,600 were unpaid as of September 30, 2013 and March 31, 2013, respectively, and are included in accounts payable and accrued expenses in the accompanying consolidated balance sheets.