Saturday, May 11, 2013

Crystal Power: Benchtop Fusion Devices

A few weeks ago I wrote several background posts on low energy nuclear reactions, (LENR).  The vote is still out on whether or not LENR is conclusively showing results.  There's another type of bench top fusion device that is working though and it's based on a crystal.  In April of 2005, three researchers from UCLA reported in Nature the results of an experiment where they fused pairs of deuterium atoms to create helium three and a free neutron with an energy of approximately 2.55 MeV[1] (picture 1).  Their experiment was more of a benchtop particle accelerator than a benchtop fusion reactor.  Utilizing the natural properties of lithium tantalate crystals, they were able to ionize deuterium atoms and then accelerate them to energies of 115 keV.  The accelerated deuterons impacted a molybdenum target coated with an Erbium/Deuterium compound.  At the energies created, the deuterons were able to fuse with other deuterium nuclei to create the above mentioned reaction.  There's some interesting physics and history behind all this.

Quantum Tunneling
First of all, classically speaking about coulomb barriers, 115 keV isn't nearly enough energy to hurl one nuclei into another.  However, as pointed out by Gamow[2] and others, thanks to quantum tunneling phenomena, particles can tunnel through the barrier with much lower energies.  This is why the deuterons are able to fuse when accelerated to only 115 keV.  Both Cockcroft and Walton's linear accelerator and then Lawrence's cyclotron made use of tunneling to perform the first nuclear disintegrations.   It's interesting to think that they might have been able to accomplish a similar feat with a much more compact setup than either experimental team actually used.  For an excellent reference on the history of particle accelerators, see [3].

Pyroelectric Crystals
The crystal used in the experiment was made of lithium tantalate. This is a type of crystal that spontaneously polarizes and develops its own electric field as its temperature is raised.  Crystals with  this property are known as pyroelectric.  The inherent electric field created by these crystals is what was used by the UCLA researchers to provide the accelerating potential for the deuterons.

The fist reference I could find to research on the pyroelectric properties of lithium tantalate was in an article from Bell Labs' researchers B.T. Matthias and J.P. Remeika.  Their article was published in 1949 which puts this particular crystal outside the reach of both accelerator teams mentioned above who were doing their work in the 1930s.

You can grow your own pyroelectric crystals using Rochelle salt[7] (picture 2).  While to the best of my knowledge you can't use them to build accelerators as described above, they do have interesting piezoelectric properties.  In junior high, I grew a few crystals  in an attempt to  experiment with piezoelectricity . If you attach electrodes from two opposing crystal faces to a small neon bulb and then bang the crystal gently with a hammer you're supposed to be able light the bulb with a voltage created by the crystal.

Rochelle salt crystals can be grown in supersaturated solution.When I was growing them, Rochelle salt was still available as a laxative in some older pharmacies in town.  Trying to get enough raw material to grow a crystal, I had more than one pharmacist in Hobbs, NM wondering what a 12 year old kid was doing buying them out of their stock of old-school laxative.


Ferroelectric Hysteresis
Because pyroelectric crystals can have an inherent electric field independent of any applied electric field, they exhibit hysteresis in an applied electric field in the same manner that a ferromagnetic material does when a magnetic field is applied.  The term ferroelectirc was coined to denote the similar hysteretic behavior and doesn't necessarily denote the presence of iron in pyroelectric/ferroelectric crystals   The hysteresis curve for Rochelle salt is shown below[5]. (picture 3)



References:
1.  Nature article on bentchtop fusion
http://dx.doi.org/10.1038%2Fnature03575
Naranjo B., Gimzewski J.K. & Putterman S. (2005). Observation of nuclear fusion driven by a pyroelectric crystal, Nature, 434 (7037) 1115-1117. DOI:

2.  Gamow on tunneling
http://dx.doi.org/10.1038%2F122805b0
GAMOW G. (1928). The Quantum Theory of Nuclear Disintegration, Nature, 122 (3082) 805-806. DOI:

3.  An excellent masters' thesis on the history of the particle accelerator by Andrew Steere
http://bt.pa.msu.edu/pub/papers/steeremsc/steeremsc.pdf

4.  Lithium tantalate as a pyroelectric, (ferroelectric)
http://dx.doi.org/10.1103%2FPhysRev.76.1886.2
Matthias B. & Remeika J. (1949). Ferroelectricity in the Ilmenite Structure, Physical Review, 76 (12) 1886-1887. DOI:

5.  Rochelle Salt properties
http://dx.doi.org/10.1103%2FPhysRev.17.475
Valasek J. (1921). Piezo-Electric and Allied Phenomena in Rochelle Salt, Physical Review, 17 (4) 475-481. DOI:

6.  More Rochelle Salt properties
http://dx.doi.org/10.1103%2FPhysRev.47.175
Mueller H. (1935). Properties of Rochelle Salt, Physical Review, 47 (2) 175-191. DOI:

7. Rochelle salt crystals
http://www.madsci.org/posts/archives/2007-04/1176763406.Ph.r.html


1 comment:

jowdjbrown said...

There's some interesting physics and history behind all this.300W AC/DC Switching Adapter