It is perfectly acceptable to give a presentation at a conference and then develop it into a full-fledged article and submit it to a journal. This will not be considered duplicate publication as long as at least 30 per cent of the content is new or different.  However, universities may have their own policies regarding the publication of a dissertation. While ideally it should not be a problem, it is always a good idea to check with your professor or department head before you make a decision.  Additionally, check whether the abstract of the presentation will be published in the conference proceedings. If you choose to publish it as part of the conference proceedings, you must cite it in your dissertation. 

Related reading:

• Can a conference paper be submitted to a journal?
• Is it acceptable to publish a conference abstract and an article based on it in two different journals?
• The basics of converting your PhD thesis into journal articles