What I learned today

Didn't see any.

t fast or slow??? )

The magnetic refrigeration people have a tendency to want to be able to rem ove it very slowly, at such a rate that the heat extracted exactly balances the heat flow into the experimental area.

I spent quite a while working out how to use an audio 18-bit DAC to generat e the slow ramps required, but the project crashed when I unexpectedly lost access to the CAD tools I'd been using. By the time I'd got the gEDA packa ge working on my home computer, the potential customer had lost interest.

Pity. The DAC was remarkably cheap, and I could get all the digital logic r equired into a single PLD. I probably could have used a couple of single-ch ip processors to do the work, but they'd have had to be synchronised. The P LD could be made to be both multi-threaded and synchronised with very littl e effort.

The low pass filters on the output took a bit of care.

The way I heard magnetic cooling, it was done with "ferroelectric" materials which had their domains aligned with an imposed field. This is then cooled by conventional means near 0 K - say 3K. Then the field is removed, the domains are free to dis-orient and absorb heat, going to .X K.

In my mind, this is an entropic effect. order-disorder. The Gibbs formula, DG = DH - TDS Delta free energy equals delta enthalpy minus temperature times delta entropy.

Note the temperature term associated with entropy.

The DG term is tied into the Boltzmann distribution of states, and the disorder change may explain the temperature lowering ability via this formula. It is like you are boiling water and absorbing heat. In the case of water, that absorbed energy goes into breaking bonds for a change of state, but in this case, the T gets lowered.

If you want to be exotic, and you live in a low cloud cover location, experiment with night radiant cooling. The night sky has a low temperature, and emitters glazed with polyethylene will cool if pointed at the sky.

Wien's formula, WL*TK about= 3000 wavelength in microns times Kelvin T is around 3000. This handy formula says that 300K emitters will have a peak black body radiation around 10 microns WL. Poly transmits there, so a cooler which requires no energy input is possible, and has been done many times.

Explanation for hoar(sp?) frost

or, better yet, a way to keep houses cool in AZ WITHOUT expending much energy! The 'average' temp is often around 90, just likes to get up to 105 a lot, but with radiant loss into the night sky, it would be possible to 'tilt' that average quite a bit and keep your home pretty constant temp for almost no running costs.

Now, if we could just figure out a way to live without water...

Frost is interesting in its variety, needles, crystals, fluff, or sheaths. The prettiest form I've seen were inch-sized very thin platelets standing upright on old snow. A field of those sparkles spectacularly in the sun, and when you walk through them, they tinkle. Made me think of 2001 - A Space Odyssey, when Bowman launches himself through HALs memory banks.

Jeroen Belleman

it fast or slow??? )

emove it very slowly, at such a rate that the heat extracted exactly balanc es the heat flow into the experimental area.

I've never done it, (adiabatic de-mag.) so I don't know. But I have done t hings where you flip a B-field around in the presence of spins. If you go slow, then the spins tend to follow the magnetic field direction. If you go fast then you can "leave the spins behind". (Fast or slow depends on the precession frequency of the spin.)

George H.

ate the slow ramps required, but the project crashed when I unexpectedly lo st access to the CAD tools I'd been using. By the time I'd got the gEDA pac kage working on my home computer, the potential customer had lost interest.

required into a single PLD. I probably could have used a couple of single- chip processors to do the work, but they'd have had to be synchronised. The PLD could be made to be both multi-threaded and synchronised with very lit tle effort.

Sorry I never heard of using ferroelectrics... I thought it was all paramagnetic salts for the low temp de-mag.

For the Gadolinium thing it's working around the ferromagnetic Curie point. I'd be fun to get a piece of Gd to play with. (I'm not holding my breath waiting for GE... when a research group says 5 years, I read that as 10-years to forever. and if it's going to take 10 years... then that is forever.)

George H.

Most companies who 'announce' something they are supposed to be doing secretly are doing it for other reasons: such as a 'placeholder' in the market, to stop someone else's product intro by making the market wait for results, and/or to obfuscate to competition exactly where they are really placing their efforts. Rarely 'testing' the market for acceptance or simply 'showing off'. And, true, you'll NEVER see whatever it is come into reality. ...my two cents

AKA "FUD". IBM was famous for this until '56.

Hoar frost is simply frost by sublimation rather than freezing.

They use swamp coolers. Only a "small" motor is needed for the fan.

Beer.

Didn't mean the 'style' of frost, meant the type that forms ABOVE freezing.

I remember the low of the night was something like 38, but the grass was covered in frost. I assumed the sky was 'cold' and froze all that stuff on it.

That's not (necessarily) hoar frost. Sure, you can have frost above "freezing" (radiative cooling) but the term "hoar frost" means something specific (sublimation).

Again, the dew can freeze, too.

got it.

True, I remember 'crunching' through the grass when it was not that cold at night.

Is there a name for something freezing above freezing temp?

Well, the ice is below freezing. ;-)

ARRRGGG! pitnicker!

enough! I give up!

But it does seem odd there is no word, or phrase, for something so readily observable.

Observable, sure but most won't believe it's possible. No, I know of no particular name for it. It's not "different" in any physical way. Hoar frost is.

In absence of some more details, it is difficult to understand what is going on. In a conventional refrigerator, adiabatic heating forces the liquid freon to evaporate and suck up heat from the coils. Maybe in this case loosing heat to the environment means the surrounding air. Then it might work.

"In India before the invention of artificial refrigeration technology, ice making by nocturnal cooling was common. The apparatus consisted of a shallow ceramic tray with a thin layer of water, placed outdoors with a clear exposure to the night sky. The bottom and sides were insulated with a thick layer of hay. On a clear night the water would lose heat by radiation upwards. Provided the air was calm and not too far above freezing, heat gain from the surrounding air by convection would be low enough to allow the water to freeze by dawn."

Today we would cover the water with a layer of cling wrap.

Superheated ice.

of the experiment (at 250ps, arguably fairly brief by most standards).

Best regards, Spehro Pefhany

No. They have it correct. The magnetic field forces order into the alloy or crystal structure spin allignement. Remove the field and the spins start shift absorbing heat as they do so. It is a part of how they get super cool materials in cryogenics. I can't see it making it into domestic refridgerators or freezes for a very long time.

Here is one of the papers that has triggered this latest story:

They have an interesting prototype material HoMn2O5 that shows a strong magneto calorific effect at 10K. That is fundamental research territory.

Gadolinium is the usual candidate for this sort of trick.