7.  Intangible Assets

The Company recorded amortization expense of $0.1 million and $0.4 million for the three and nine months ended December 31, 2013, respectively. Intangible assets consisted of the following as of December 31, 2013 (in thousands):

The Company recorded amortization expense of $0.1 million and $0.4 million for the three and nine months ended December 31, 2012, respectively. Intangible assets consisted of the following as of March 31, 2013 (in thousands):

Expected future amortization expense of intangible assets as of December 31, 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 $15,000 and $19,500 were earned by Solar for the three months ended December 31, 2013 and 2012, respectively. Royalties of approximately $62,300 and $52,100 were earned by Solar for the nine months ended December 31, 2013 and 2012, respectively. Earned royalties of approximately $15,000 and $24,600 were unpaid as of December 31, 2013 and March 31, 2013, respectively, and are included in accounts payable and accrued expenses in the accompanying condensed consolidated balance sheets.