I am trying to figure out the best way to join some 12 gauge wire that will be carrying very high DC current (about 100A) for about 2 seconds. When working with lower currents, soldering seems to be the best way to go. But, with such a high current I am unsure of the best way to proceed. I found here that a bus bar may be the best way, but I am working in a very tightly restrained space and I do not think there would be room. Any help is greatly appreciated. EDIT: I failed to mention that it is running a voltage around 40V
You will need some crimp-on connector like these.
Use the right crimping tool! I insist on that. Hexagonal crimp is the way to go. As you can see in the image below, the result is basically a solid mass of copper, all cold-welded together. Really tough, low-resistance, this is what you want.
More info here on other crimping styles which are much easier to screw up.
Are you going to make and break the circuit while 100A are flowing? If not, some type of crimp-on connector will work fine. For example a butt splice crimp connector. These are available at hardware stores, online, etc. There is a color code scheme commonly used. You want the yellow color for 12AWG wire. Ideally you should use a ratcheting crimp tool. Give the crimp a solid tug to make sure the wire does not slip at all. If you can pull it out with a jerk or tug, the crimp was no good in the first place.
Note that 12AWG wire is not even remotely adequate for 100A continuous duty. The two problems you will have are that there will possibly be substantial voltage drop (depending on how long the wires are) and the wire may heat up quite a bit if you let the current continue for more than the 2 seconds that you mention in your question. So make sure that doesn't happen.
If this apparatus or whatever needs to pass any kind of safety test or inspection, or if it will be operated by unqualified people, you may want to consider using heavier gauge wire. You could also use special wire with very high insulation temperature rating to help reduce chances of disaster. E.g., silicone wire.
Since you haven't said much about what you are doing, I just want to make clear that you assume all risk. My advice may not be very good for your particular situation due to information not disclosed, etc. So be careful when you first bring this online. It is up to you to make sure it is safe. 100A will give you a very energetic spark, and can easily spot weld contacts on switches etc.
The classic solution is called a 'wire nut'; this forces two wires into contact with a tapered tube that has screw-threads inside. The current goes from one wire to the next and not through other metals. like these
Unless the voltages are very low, copper needn't be cleaned or deoxidized, a thin copper oxide will not impede conduction.