Is there a gas or a mixture that be safer then hydrogen that can be used to replace helium in a lighter then air ship, such as a zeppelin?
Hot air?
I don’t think you’re going to find anything besides hydrogen and hot air, and hydrogen is flammable. The next noble gas up is neon, with a density of 0.9 g/L (compared with 0.18 for Helium and 0.09 for hydrogen. Normal air has a density of 1.2 g/L, so neon is lighter than ambient air, but not by much, and not enough to be useful in lifting anything. The other noble gases are heavier than air, with argon clocking in at 1.78 g/L.
The next element up after helium is lithium, which is a solid at room temperature. In factm, it’s a metal, and a pretty reactive one. The compound lithium hydride, LiH, is a whitish powder that doesn’t float, although usually LiH is a dirty black or brown from impuritioes. I once had a crystal of Lithium Hydride, which was a gorgeous light blue. I had to keep it under oil, though, because it’s reactive (LiH plus water releaszed hydrogen gas – they used to pack it as “storable hydrogen” for emergency rescue balloons). It was so light that it floated in oil. In fact, it would float in anything. LiH is, I believe, the lightest solid in its natural form (you can get lighter aerogels and the like, but LiH is the lightest “non-engineered” msaterial, with the lowest natural density). But you’re not going to fill a zeppelin with it.
If you go up a bit, you get Lithium Fluoride, which is also pretty light. It’s transparent farther into the ultraviolet than any other solid. Although it’s nominally an alkali halide salt, it’s remarkably resistant to dissolving in water, it’s very hard, and takes an excellent polish. When it has specific defects in it, the crystal makes several cool lasers.
An excellent fireproof alternative to helium is a vacuum, or any inert gas as very low pressure. Then you just have to find a way to contain it.
Methane has occasionally been used, but that’s flammable, too. It DOES have the advantage of not diffusing through envelopes as easily.
Lift gases, according to wiki:
Hey, I know … we just invent an industrial process that pollutes the atmosphere with sulphur hexaflouride, then anything will work …
A vacuum or gas at low pressure isn’t, by definition, a lifting gas (e.g. one that provides lift at ambient pressure). Some lifting gases, like hot air, provide buoyancy at elevated temperature, but at STP the only other gases besides hydrogen and helium that are really well suited are methane (which is, of course, flammable) and carbon monoxide (is not, but is relatively heavy). There are a few other gases that are marginally buoyant in air such as nitric oxide, but these are even less desirable than hydrogen due to toxicity and reactivity (in addition to flammability).
The main problem with hydrogen as a lifting gas, by the way, isn’t the flammability but the permeability of the gas in most substances that would form a container. Hydrogen can escape from the smallest leak, and will migrate through materials that would be virtually impermeable to other gases, even helium. It also will react with many common metals causing undetectable embrittlement. As such, any systems that contain or interact with hydrogen have to be specially certified and protected. Of course, when hydrogen permeates an already flammable substance it also makes it more prone to combustion by dramatically lowering the initiation temperature. But the flammability hazard of pure hydrogen, and the resultant overpressure energy is overrated, especially since any escaping gas will go straight up in atmosphere.
Stranger
Carbon monoxide is flammable. It doesn’t matter if a vacuum or low pressure gas is considered a lifting gas. And hydrogen doesn’t migrate through materials as readily as helium does.
I like these:
Now I want a balloon filled with diborane. And a really long string for it.
I like these:
Now I want a balloon filled with diborane. And a really long string for it.
Edited to add: If I go by the listings on the Wikipedia pages, diborane is actually slightly heavier than air, and is so close in any case that I can’t see it having any effective lift.
Hot Diborane Balloon!
Is there something that could be mixed in with hydrogen that could prevent ignition but still not be so dense as to render the hydrogen useless for lifting?
Helium has been proposed for that.
Experiences with an ammonia balloon:
http://www.balloonlife.com/publications/balloon_life/9703/freeflt.htm
Since the OP specifically said “fireproof”, and not “safe” and certainly not “cheap”, I propose antihelium as an alternate.
Is helium all that flammable? I figured it wouldn’t burn since it’s a noble gas and doesn’t react with anything.
Unlike hydrogen, helium is scarce, expensive, and non-renewable.
Carbon monoxide is inflammable at STP, as it has an autoignition temperature at 882 K. A lifting gas, by definition, is buoyant at ambient temperatures. An aerostatic device such as a balloon or dirigible relies on the gas to be at or slightly above ambient pressure, contained by a textile skin held in tension by pressure. A low pressure or vacuum container can be buoyant only if it is sufficiently rigid to maintain integrity against buckling due to the pressure difference and if the net density (of the pressure vessel material and empty volume) is less than that of the ambient pressure. There is no practical material than can meet these requirements for any useful lifting capacity.
Hydrogen by itself will not ignite. I have done the demonstration of inserting a candle into an upside-down flask and had it extinguish because it was filled with pure hydrogen gas. But when it escapes or comes into contact with an oxidizing compound it can combust.
Stranger
I know this doesn’t specifically answer the OP’s question about lifting gas, but I think it needs to be seconded.
The trick, in my mind, is a very light, relatively strong foam. Something like aerogel, but designed with open cells so that you can suck the air out of it after formation. The foam would help solve the problems of building something strong enough to contain a vacuum, would probably permit lots of little vacuum chambers instead of a few big ones, and would slow down the loss of lift if there was an air leak.
But within the OP’s airship context . . .
It’s a tough materials problem. Maybe practical someday, but not now. Nothing near total vacuum is necessary, but the pressure differential is a big problem. Lighter than air craft aren’t pressurized like a toy balloon, and a small leak gives a chance for the craft to descend at a reasonable rate. A leak in a in a low pressure vessel will probably cause an instand implosion, or rapid re-pressurization. I’d rather be carried aloft by a balloon with a flammable gas (except maybe the diborane) than any kind of vacuum balloon for the time being.