ramdisk /tmp

The /tmp directory is a RAM based directory. Means that it doesn't wear out like an SD card, or so...
Obviously when rebooting, you lose all the content. It still is the most obvious place to put temporary files, etc...

To make it look like a seperate drive you can do:

mkdir /tmp/ramdisk

in rc.local.
If you want it to look like a mounted drive you can also

ln -s /tmp/ramdisk /mnt/ramdisk