There is an expected research paper voice and while the mode is driven by organization, voice still plays a part in this form of writing.
I'd describe the desired voice in this mode as; authoritative. Other descriptors would be strong, confident, and understanding. Lovitts 2007 p 36
It comes down to understanding the audience for the writing. There is a very clear and defined audience for research papers: other professionals or academics. Knowing the audience helps a student writer determine the voice (once they have the concept of voice in place).
You can help your students understand voice as a concept, and your expectations for a 'research voice' by using contrasting examples. (Here's where a search for appropriate mentor texts will help.)
For instance, you could provide strong and week thesis statements.
Also find or create contrasting examples. Demonstrate overly casual, or insecure voice, and contrast it with confident (fact supported) academic voice.
On one hand, you have an example of clear well supported, and authoritative writing.
Contrast this with excessively elaborate, vague and uncertain writing.
Examples of students attempting to hide a lack of understanding and content with elaborate long winded and confusing sentences could go a long way toward helping define the concept of ' research voice '
This reinforces the need to tune the writing to the audience.
You could also contrast the research piece with an expository piece to show differences in voice based on mode of writing.
While the voice of a research report is more constrained, it's there. By helping your students learn to recognize voice you clarify the intent of the writing and help build the concept.
Lovitts, B. E. 2007 Making the implicit explicit: creating performance expectations for the dissertation. Virginia, Stylus Publishing