A few words of warning when searching:
- Not all of the abstracts are in the same language as the dissertation. Only once you click on the title and get to the page about that dissertation can you view the "language" section. Some of the English dissertations have abstracts in Greek or German, and visa versa.
- Not all of the dissertations are open access. The one that looked most interesting to me denied me access. Sad. However, this still looks to be a great resource.