Carbon dioxide has no liquid form. At high enough temperature and low enough pressure it sublimates, that is, it changes directly from a solid (dry ice) to a gaseous form. To build a refrigeration unit you need a non-acidic and non-flammable gas such as chloroflouromethane (R-22) or tetraflouroethane (R-134A) that will liquify at a suitable pressure in a condenser and give you the temperature drop you need across an expansion orfice. The refrigerant must be molecularly compatibile with the oil used to lubricate the internal components of the compressor. This assures a stable and long lasting refrigeration unit.