To have some fun with superconductors I am planning to buy them from here.
But it requires me to cool liquid nitrogen to a temperature of 77 degrees Kelvin.
Can some one tell me what kind of refrigerators (most economical) do I need to purchase to achieve this ? Or Can I get liquid nitrogen which is already cooled to less than 77k?
Another doubt is the liquid nitrogen is already cooled to 77 K?
There is at least one tutorial on the web that illustrates a means by which one can build a device to condense one's own nitrogen. I haven't tried to duplicate this setup, but the principles seem sound. It essentially uses regenerative cooling by exploiting the fact that gasses cool when the pressure is suddenly lowered (via the Joule-Thomson effect). Be aware that this project uses both high pressure (200 atm) and extreme cold, so appropriate safety measures are extremely important.
You can buy liquid nitrogen, it generally costs a $1/litre but as you need a fairly large refrigerated tanker truck to deliver it in - they tend not to sell small amounts.
A local hospital/university lab would probably be happy to let you have a litre, it's easily carried in a thermos flask, unless you live in a country heavily infested with lawyers.
edit: Sorry this wasn't really a serious answer!
Liquid nitrogen is widely available and used in hospitals, university labs and industry. It typically costs roughly the same price as bottled water in quantity. Unfortunately nobody is going to deliver you a single litre. It is delivered in heavily insulated tankers and stored in large insulated tanks on site. You can keep small quantities in a regular thermos for an hour eg when we do demonstrations to schools. It's not the most dangerous substance in the world but there are hazards which is why nobody is going to give you any to play with.
There's a fairly cheap tutorial from instructables: Homemade Liquid Nitrogen Generator (Instructables)
In general you don't condense your own nitrogen in small amounts. If you have an industrial-scale refrigeration plant you can profitably sell already-liquid nitrogen for about the same price per liter as milk. If you drive around town looking at the loading docks behind university buildings, hospitals, maybe machine shops, you'll see a little fenced area containing a tall white cylinder with cryogen hazard shipping marks on it. That's the dewar they get filled from the compression company, and those are the folks you might talk to about getting a few liters at nominal cost (perhaps even for free, if it's a one-time exchange and they're not set up for sales). dolphus333 suggests welding supply shops might carry nitrogen (and in that case would also carry appropriate tools). Poking online reveals that cattle breeders use nitrogen-cooled containers to transport bull semen, so big agricultural suppliers are a possibility, too.
A proper dewar with a reasonable capacity is more expensive than you might expect. An all-stainless steel thermos bottle is essentially the same thing. I don't think I'd trust a thermos with a glass interior not to shatter when filled, though if it survives that initial thermal shock it's fine until the nitrogen boils away. In any case, don't tighten the lid! There's an enormous amount of thermal energy in the room-temperature atmosphere slowly seeping into cold nitrogen, and the pressure of the boiled-off liquid will be higher than the strength of any lid; when the pressure gets high enough to break the lid, the resulting explosion is quite dangerous. I'd recommend drilling a hole in a tight-fitting plug, or throwing it away, so that you don't forget. Proper dewars come with foam plugs.
I have seen designs for homebrew flasks made of soda bottles wrapped in styrofoam. If you make one of these, drill a hole in its lid so that you can satisfy your instinct to put a cap on without putting yourself in danger. I personally don't trust these: boiling nitrogen (77 K) can condense oxygen (at 90 K) out of the air. A little bit of liquid oxygen is dangerous around carpet sparks, and I always feel a little static when I handle styrofoam, and most of these designs have little cold air gaps where condensation might collect. Might get you an interesting story, though.
Buy a pair of heavy, loose-fitting garden gloves so you can hold cold surfaces tighter and for longer without hurting yourself. Very brief contact is okay — you can actually dip your finger in the liquid without harm (like "bloop, oh, okay," but not much longer).
Resist the urge to store your nitrogen in your kitchen freezer. It's only about a 15% change in the temperature difference from your 77 K nitrogen to your 270 K freezer to your 300 K kitchen, so it won't buy you that much extra time. The nitrogen will cool the freezer much more effectively than the freon pump that the freezer comes with. Depending on how much nitrogen you have and how thermally leaky your flask is, you might actually freeze and solidify the working fluid for your kitchen freezer — an expensive mistake. If you'd like another layer of insulation when you end your experiments for the night, use an flip-top picnic cooler with a loose-fitting lid.
I apologize for only touching on your actual question — where to buy liquid nitrogen — and giving you a long safety lecture. Your confusion about whether liquid nitrogen is a thing to be refrigerated or a refrigerant suggests you are quite inexperienced.
If you have a lot of fun with your nitrogen-cooled superconductors and decide to upgrade to helium, don't waste your time asking on web forums — find a real expert and buy him dinner. Helium is weird.
Call your local welding supply store. They generally have some. If you live in a small enough town and they want to deal with you, they'll be happy to sell you some. Be up front, tell them what you intend to use it for and how you're going to handle it. If you're using it for school or science fair, this typically paves the way. It will evaporate fairly quickly depending on what you bring to carry it in. Insulated containers work better. As it evaporates, it will develop a dangerous pressure in a container with a tight lid. The explosion in the video below is caused by a one litre coke bottle full of liquid nitrogen with the lid on tight. Also, don't spill it on yourself. Read up and be careful.
