Links below are for technical report citations and some online PDFs.
From the Department of Energy (DOE). For best results, sign up for a free account and use the Advanced Search. 1943 - present.
Also, try the DOE OpenNet System.
National Technical Reports Library (NTRL)
Contains PDFs of many reports. For better search capabilities, try NTIS.
NASA Technical Report Server (NTRS)
Abstracts and full-text reports of NASA and NACA, 1915-present
TRAIL: Technical Report Archive & Image Library
Includes U.S. Atomic Energy Commissions (AEC), U.S. Bureau of Mines, U.S. Dept of Commerce, National Bureau of Standards. Most reports prior to 1975.
David Taylor Naval Ship Research and Development Center Reports
Selected reports from DTNSRDC series C, R, & Trans.
Nuclear Regulatory Commission (NUREG)
Some are available in Barton. Try a "Title Begins with" search for the series number. Example: NUREG/BR
Atomic and physical properties of the elements; atomic and molecular spectroscopy; nuclear physics data; condensed matter physics data; and more. A searchable element index combines the data from various sources.