5. Intangible Assets

        Intangible assets consisted of the following (in thousands):

        Amortization expense for the intangible assets was $0.5 million, $0.5 million, and $0.8 million for the years ended March 31, 2014, 2013 and 2012.

        Expected future amortization expense of intangible assets as of March 31, 2014 is as follows (in thousands):

        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 $83,000, $76,700, and $72,800 were earned by Solar for the years ended March 31, 2014, 2013 and 2012, respectively. Earned royalties of approximately $20,700 and $24,600 were unpaid as of March 31, 2014 and 2013, respectively, and are included in accrued expenses in the accompanying balance sheets.