Friday, January 23, 2015

Lead Sample Purity and Tube Cutting: Lab Book 2015_01_22


For background on the experiment, please scroll down.

We can get a cheap piece of 3/4 inch diameter Pb from Rotometals.  Here are the details
3/4 17.14 per foot
Nuclead has the same thing.  Check for purity and price.
Also there’s Mayco.
Next question, how pure does pure have to be?

Pb purity data:
The following are reference articles about superconductors.  Each of them describes the purity of the Pb samples used.  The lower bar is set by the RMP article referenced below, as well as one other that mentions the use of ‘commercial’ grade material and the evidence of a transition to a superconducting state for this material.  The final reference from 1886 in the section defines commercial level material to have a quality not lower than 99% pure.  This information is being researched to determine what purity of sample we should use.  It would seem that a higher purity sample will produce fewer unexpected experimental results, as well as fewer alternative hypotheses to track down.  There is, however, a trade-off with the cost of the sample.
The Specific Heat of Lead in the Temperature Range 1'K to 75'K*, M. HoRowITz) A. A. SILvIDI, S. F. MALAKER) AND J. G. DAUNT  99.99% pure

Equilibrium Curve and Entropy Difference between the Supraconductive and the Normal State in Pb, Hg, Sn, Ta, and Nb, J. G. Daunt and K. Mendelssohn, Proc. R. Soc. Lond. A 1937 160, doi: 10.1098/rspa.1937.0099, published 1 May 1937  purity > 99.999
Look at 15% field lines frozen in Pb:  free version of the article:
The frozen in field lines are hypothesized to be due to the presence of more impurities in Pb, (15% frozen in lines),and Sn, (~10%), than in Hg, (0%), which “can easily be prepared in a state of extreme purity.”
This article references an article that describes the phenomenon of frozen-in field lines and how to detect them:

It was also commented that the sharpness of the transition from the normal to the superconducting state appeared to be dependent on the number of impurities.  Fewer impurities led to a sharper transition from normal to superconducting.
Section 4.i. of this article:
Section 4.i. discusses the freezing-in of flux lines again.  A dependence on the purity of the material is mentioned.  The expulsion of flux lines spontaneously without a change in flux, as often mentioned by Hirsch, is discussed in section 4.e. 
Notes on the impurity of ‘commercial tin’ used in the experiments described in the RMP article can be found below in an excerpt of a book apparently about making tin cans:
It sounds like the metal was in fact fairly impure, whatever that means quantitatively.
Here’s more information on the purity of metals.  The N numbers, of purity are discussed.  3N is 99.9 percent pure.  4N is 99.99% pure.
Here’s a quote on the quantitative quality of ‘commercial’ tin:
“In the United States, the purity levels for commercial grades of tin are defined by the American Society for Testing Materials (ASTM) Standard Classification B339. The highest grade is AAA, which contains 99.98% tin and is used for research. Grade A, which contains 99.80% tin, is used to form tinplate for food containers. Grades B, C, D, and E are lesser grades ranging down to 99% purity. They are used to make general-purpose tin alloys such as bronze and solder.”
Here’s the source
From page 625 of the following, written in 18xx, closer to the time of the RMP article in 1935
“Purity of commercial tin—The tin of commerce generally contains a very small quantity of iron and lead and traces of other metals, but rarely exceeding 1 percent of impurity.”
Random Notes
On glass Dewars

Sample Stage Preparation
Work was continued on building the sample state.  The Dewar stick was separated from its attached brass platform using a tubing cutter.  The aluminum stick was very soft and the first cut resulted in the stick becoming crimped.  A second cut was made more successfully at a position above the one crimped by the initial cut.  Care was taken not to cut any of the wires travelling through the tube.


Initial Crimp



Final Cut



Background
Hirsch's theory of hole superconductivity proposes a new BCS-compatible model of Cooper pair formation when superconducting materials phase transition from their normal to their superconducting state[1].  One of the experimentally verifiable predictions of his theory is that when a superconductor rapidly transitions, (quenches), back to its normal state, it will emit x-rays, (colloquially referred to here as H-rays because it's Hirsch's theory).

A superconductor can be rapidly transitioned back to its normal state by placing it in a strong magnetic field.  My experiment will look for H-rays emitted by both a Pb and a YBCO superconductor when it is quenched by a strong magnetic field.
This series of articles chronicles both the experimental lab work and the theory work that’s going into completing the experiment.

The lab book entries in this series detail the preparation and execution of this experiment… mostly.  I also have a few theory projects involving special relativity and quantum field theory.  Occasionally, they appear in these pages.

Call for Input
If you have any ideas, questions, or comments, they're very welcome!

References
1.  Hirsch, J. E., “Pair production and ionizing radiation from superconductors”, http://arxiv.org/abs/cond-mat/0508529

No comments: