Why "Cryo" anything?

Hi guys...just joined..

Cryo treatment of steel: As steel is cooled to a point below M s , known as the martensite start temperature, martensite will begin to form, it being transformed from austenite.. At M f , a lower temperature known as the martensite finish temperature, all of the austenite will have transformed. This gap in temps exists because the lattice changes that occur stabilize the structure are volumetric ones...austenite is FCC while martensite is BCC...(face center cubic vs body centered cubic.)

Varying the cooling steps and temperatures can force pearlite and martensite to exist together, ratios and relative amount modified by the temp profiles...What is consistent here, is that all the treatments alter the composition at the lattice level. The cryoing of steels, brasses, et al, do indeed cause a re-structuring of the crystal structure, as the strain induced by the lower temps will be reduced by molecular adjustments...some of those adjustments alter the marco physical properties...these changes are used in many areas, such as engine stuff, turbine blades, knives...etc.

Cryoing of copper, however, while theoretically capable of adjusting the crystal lattice (I've not seen this, btw.), will not significantly alter the mean free path of the electrons at room temp (this is the energy loss we call resistance). It will still be at the 3 times 10 -6 regardless..there are no discontinuities caused by grain boundaries, no signal reflections...nada.

A polymer below T g is indeed a solid, and it's TCE is a specific value..for example, E+C 2851 will be about 29 PPM/C. Above T g , TCE will be in the 90 to 120 PPM/C range, and will be in a plastic (soft) state.

At nitrogen temps (77K), kapton and tefzel are still quite flexible, and kapton only at helium temps of 4.5K and 1.8K.

AT 77K, only bisco and yibco are superconductors..too brittle for use yet, and J c (critical current) are not very high, but they are getting better.

Adhesives do not melt at cryo temps..If thin enough, they will be ok, unless they are being used to bond differing TCE materials, for example..copper (16PPM) to aluminum (25PPM), then they will fail in shear.

Adhesives and epoxies that are greater than 20 mils in thickness will crack if they are bonded to a metal, the thicker the adhesive or epoxy, the higher the temp they will fail at.

At the 5 to 10 mil thickness, unfilled epoxies will work and remain quite strong all the way to superfluid helium (1.8 K). However, even by themselves unbonded to anything else, they will tend to craze and crack if they are not cooled down extremely slowly, as the combination of heat capacity and thermal conductivity do not allow high cooling ramp rates.

Jena labs stated "the LN2 is actually much colder than this temperature."...they meant room temp..not below 77K. To get below 77K, they have to pull a vacuum. Not very hard to do, but still requires some horsepower..with helium, a reasonable setup will require several hundred horsepower..some do this with nitrogen, to get measurements at 50K.
"Micro-diodes" do not exist..period.nor does "slow field transverse energy generation"..skin effect is a current slew rate based entity, relying entirely on the radial conductivity of the wire, the geometry of the conductors, internal magnetic field rate of change, and the ability of the surrounding dielectric to charge the internal volume of the conductor with magnetic field. If you are looking for 50 hz signal propagation at 2.93 feet per second, you are looking in vain..

I was unaware silicon could be damaged by cryo..that has not been my experience...you intended to say silicone, perhaps??

Cheers, John

I mentioned the martensite transformation for one very important reason. It is a "diffusionless" phase transformation..in other words, the change at the atomic level, from a face centered cubic crystalline structure (FCC) to the body center cubic one (BCC) will occur without the need for specific atoms to migrate within the structure. So, all the atoms are there, they just re-arrange a tad. As the metal is cooled, the driving force that makes the structure change to BCC eventually is large enough that the changes occurs.

Diffusion based transformations on the other hand, do indeed require heating, so that the atoms have enough energy to move about to where they want to be..a good example is that of a "cored microstructure" like copper-nickel, where initial solidification is around particles of one alloy, and subsequent solidification has a gradient of alloy content..this structure requires heat treatment to homogenize the overall material. BTW, all of this stuff is in Barrett, Nix, and Tetelman.."The principals of engineering materials",Prentiss Hall, 1973..

Obviously, for a cryo treatment to work, the transformation would have to be a diffusionless one.. Since cryo treatments are shown to be effective for modifying macro properties for many disciplines, one can certainly make the argument that there are many different diffussionless reactions out there.

For wires, I certainly cringe at the thought that the metal undergoes some "magic transformation" which somehow makes it easier for the electrons to glide through the lattice, and certainly would expect to measure any such change as a change in resistance. Plastics, on the other hand, are more difficult to brush off..

CD's and cryo for example..I'd look at:

1. Does the process stress relieve the plastic, making the disk flatter as it is spinning? (the internal stresses of a spinning disk will be slightly different from one at rest).
2. Does the process help re-arrange surface atoms in either the reflective layer or the poly surface?
3. Does it alter the optical properties of the poly by surface re-arrangement?. or, perhaps some diffusionless transformation similar to martensite?

I would assume that all these could be easily checked by checking the end result...bitstream comparison of two cd's, one cryoed..

Bill..a distinction must be made between intelligence and work experience..I have some experience in the cryo world..but thanks for the kind words...

Cheers, John

Yes, you are correct in part. In order for the transformation to martensite to occur, austenite is indeed required..and that is obtained by heat.

What is more important, though, is the fact that it is a diffusionless process, one that does not require heat treatment for it to start..it requires the driving force, which in the case of the change from austenite to martensite, is not an increase in temp, but a decrease to below the martensite start temp.

The fact that cryo is indeed used to alter the macro properties of any material means that one is using a diffusionless process..and, it does not necessarily require a pre-treatment to higher temperatures first..The argument that all metal objects require heat to actually form them, before one can cryo them, is just a semantic one. There are processes that do not require the end manu heat them prior to cryo..

But, a diffusionless process does not require a heat precursor, but instead, use the internal lattice forces being created by the cooldown..that is the driver force..not heat..

Some quotes:page 311, same text..

"T3 is so far below the equilibrium transformation temperature Teu, that the driving force for FCC austenite to transform to BCC ferrite is enormous.....

"The diffusionless transformation by which (greek symbol meaning austenite phase, no html codes here) decomposes to martensite takes place by a complicated shearing of the () lattice. Each atom moves only a small distance relative to it's neighbors, less than one atomic distance. Consequently, thermal activation in the sense of vacancy motion or solid state diffusion is not required for the formation of martensite.

Since martensite formation is a diff. transformation, it cannot be supresses by quenching and, irrespective of time, a certain amount of martensite will form at a given temperature...the amount of martensite that forms at a given temperature will increase with increased cooling...at a temp Tf, all the austenite will have transformed."end of quotes..

So, clearly, the diffusionless transformation equilibrates at any temperature between the start and finish temp. If the object is, at a later date, taken down to a lower temperature, more martensite will form..this will continue until all the austenite is gone...

Now, the real question, is...is this type of diffusionless process possible with plastics?..I don't know.

Cheers, John

""The driving force for the martensitic transformation is the instable crystal structure of Austensite at lower temperature. So without forming austenite again. The driving force is gone.""

The point is, at a specific temperature, austenite/martensite ratio will be stable, and lowering the temp changes that ratio to another, stable one..eventually, a temp is reached where all the austenite is gone.

Now, purchase a material, it is delivered, and in your hand..next, assume that what you have in your hand is not fully stable at the atomic level, and if you lower it's temp, a diffusionless process stabilizes the lattice..that is what the cryo process is all about..yes, heat was initially involved, but after you got the material, all you did was cool it..

It's that instability I'm talking about..the austenite to martensite transformation was the easiest example to use to explain diffusionless transformations..

It's the concept of a transformation that occurs as a result of cooling something that is important..

Cheers, John.

PS..be back in a week..on vaca...it's been a pleasure..