Perhaps you should try some spelling variations:
"Improving thermal performance metrics." » "Improving thermal performance measures."
"improving human performance series." » "improving human performance studies.", "improving human performance size.", "improving loan performance series.", "improving chain performance series."
improving thermal » improving external, improving animal, improving erm

You may be able to get more results by adjusting your search query.

• Performing a fuzzy search may retrieve terms with similar spellings: (("improving human performance series.") or ("Improving thermal performance metrics."))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: (("improving human performance series.") OR ("Improving thermal performance metrics."))*.
• Removing quotes may allow a broader search: (( improving human performance series. ) or ( Improving thermal performance metrics. ))*.