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In English-language papers, I often see "dc" (as in direct current) in lowercase, but "RF" (as in radio frequency) in uppercase. For example, this paper includes both usages at the bottom of page 618:

This means that the ac (RF) waveform applied to [...]

Some more papers using "dc".

• 
  http://instructor.physics.lsa.umich.edu/adv-labs/Mass_Spectrometer/MassSpecQMS.pdf


• 
  http://www.vacuumcursus.nl/casussen/jcequad.pdf


• http://ir.gig.ac.cn:8080/bitstream/344008/13774/1/08018.pdf

Oddly, this one uses lowercase "dc" and "rf":

• 
  http://www.stanford.edu/group/Zarelab/publinks/798.pdf

Where does this convention stem from?

The American Institute of Physics Style Guide (4th edition) has a table of common abbreviations. (http://kmh-lanl.hansonhub.com/AIP_Style_4thed.pdf)

Alternating-current, direct-current and radio-frequency are all abbreviated in lower-case in this guide.

I'll make one comment on all three of these abbreviations and their use.

These terms are often used in sentences where their expansion would not make any sense. Whether this is acceptable use or not is a matter for debate, as these abbreviations carry meaning which goes beyond their original (narrow) definition.

In particular the concept of a time-varying or constant signal is often described/explained using 'ac' or 'dc' despite the signal concerned having nothing to do with electrical current (alternating or direct). For example you often hear the term 'dc' being applied to the first term (representing the mean) in a discrete Fourier transformation (e.g. FFT) of some (arbitrary) signal.

The term 'rf' (radio-frequency) is used to describe the physical phenomenon (wireless electromagnetic radiation) rather than the frequency of that phenomenon. e.g. you might have an 'rf communications link' which would mean some kind of wireless radio link as opposed to a communication link that uses a cable (despite both potentially using the same frequencies).

Besides my current occupation as computer programmer (where acronyms and initialisms seem to be capitalized with no discernible convention), I have been an electronics technician and am a Ham Radio licensee. I would never write "dc" for direct current. This would be DC, always. But let me give a really big caveat to this.

It may be that there is an emerging or emergent convention with "dc" and "ac" (for "alternating current") that I have missed. I just checked the most recent edition of QST, the magazine for Hams, and find (to my surprise) that in referring to voltage they are using "dc" and "ac"! At least when using them as part of a measurement, such as 100 Vac or 100 Vdc. However, I definitely remember always seeing these written as "AC" and "DC". At least when I was first learning electronics back in 1966.

If QST is doing it, and the papers you mention are doing, then it seems very likely that for these terms there is now a convention to lower-case them.

Lower casing in abbreviations does occur already, of course. In a technical realm, where multipliers such as "milli" and "mega" occur, it would not be useful to allow "MV" to stand for both millivolts and megavolts! A thousandth of a volt and a million volts do differ in size, after all. So we do "mV" and "MV".

IEEE 100 says to use lowercase.

I think the IEEE recognized frequent confusion between Air-Conditioning, or Aircraft (often abbreviated A/C or AC or even "HVAC"). So, "ac" doesn't conflict with any of these.