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Frequently Asked Questions

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Purity FAQ
Why do we need high purity metals?
How pure are Ames Laboratory's rare earth metals?
When discussing purity, what do you mean by 5N or 3N?
What is the basis for purity?
What is the difference between atomic and weight based reporting?
What is the measurement error?

Stoichiometry FAQ
How do you achieve precise stoichiometry in alloying?
Will the true stoichiometry please stand up?

Conversion FAQ
How do I convert atom percents to weight and volume percents?

 


Why do we need high purity metals?

The effect of impurities on alloy properties is well established (1) but is largely forgotten in discussions about bulk materials. The bottom line is this: without high purity metals, researchers may be missing, or not seeing, the true physical properties (or physics of the material) which results in missed opportunities to refine theories, models, or develop devices.  Let the image below serve as an example why purity is a critical factor in fundamental research: 

Available Entropy effect by impurity

The obvious difference in the two entropy curves for the two Gd5Si2Gesamples is due to purity. The large peak in the high purity (KAG) sample is derived by both a magnetic and structural transition. This structural transition is not exhibited in samples made from low purity metal.  

Another example of purity driving properties in superconducting MgB2 is found in reference (2).

(1) Metals, alloys and compounds-high purities do make a difference!
Journal of Alloys and Compounds, Volume 193, Issues 1-2, 15 March 1993, Pages 1-6
K. A. Gschneidner Jr.

(2) Effect of Boron powder purity on superconducting properties of bulk MgB2
Physica C: Superconductivity, Volumes 460-462, Part 1, 1 September 2007, Pages 602-603
Xun Xu, Dayse I. dos Santos, J. H. Kim, W. K. Yeoh, M. J. Qin, K. Konstantinov, S. X. Dou.

(3) Analytical Techniques in the Sciences, Chapter 1: Analytical Measurements, Edited by Graham Currell, (2000) John Wiley & Sons, Ltd.

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How pure are Ames Laboratory's rare earth metals?

Please examine the image below:

Purity of Lanthanum vs periodic table

The above chart is based on a comparison of Ames Laboratory lanthanum vs. a 4N (99.99%) commercial lanthanum product.  In terms of atomic percent, both are very good with respect to intra-rare earth impurities (orange highlighted elements), with the Ames material being slightly improved, as shown in the orange highlighted values.  If we add in alkaline, alkali, transition metals, metalloids and some of the non-metals (yellow highlighted elements), the Ames lanthanum is still a 4N metal while the commercial is a 3N metal.  Now add the gasses (the green highlighted elements) and the Ames metal is 3N while the commercial material is just breaking a 2N rating.  This type of relation holds for Ames and commercial materials starting at 5N rare earths purity.

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When discussing purity, what do you mean by 5N or 3N?

It is important to understand how purity is defined when acquiring alloying materials. When a material is declared as 99.99% pure, or 4-nines, or 4N, the declared value and the true value of the concentration of impurities most likely are not the same.  This is due to many manufactures not testing for all possible elemental impuirties.

Let us first lay out what the % or Nines scale means in terms of parts-per-million (ppm)

Nines Purity % Total Parts Matrix ppm Impurity ppm
1N 90% 1,000,000 900,000 100,000
2N 99% 1,000,000 990,000 10,000
3N 99.9% 1,000,000 999,000 1,000
4N 99.99% 1,000,000 999,900 100
5N 99.999% 1,000,000 999,990 10
6N 99.9999% 1,000,000 999,999 1
7N 99.99999% 1,000,000 999,999.9 0.1
- 100% 1,000,000 1,000,000 0

So, for example, in a 4N pure tin (Sn) ingot, for every million atoms of matter,  999,900 of the atoms are Sn atoms with 100 atoms of other elements.

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What is the basis for purity?

The issue of purity is often confusing, as there is the absolute purity and the common metals basis purity. On an absolute scale, 99.999% is a very high bar to achieve, since this allows for only 10 ppm impurity with respect the all elements on the periodic tableMetals basis, as is commonly indicated in catalogues, does not include many of the metallurgically important interstitial elements or other nonmetallic impurities. For example, a certificate of analysis that declared Mn is 99.95% purity (min. metals basis) also reported that the Mn has 0.3% oxygen. The Mn is not 99.95% on an absolute basis. If you include the oxygen content, at best the Mn is 99.65% pure on an absolute basis. A metals basis purity may not include: H, B, C, N, O, F, Si, P, S, Cl, As, Se, Br, Te, I, At. Some certificate of analysis forms may only list a handful of elements that were tested; the remainder are untested or unreported. The true value of the impurity concentration is therefore not known.

Why are such limited assays reported? Because vendors are pragmatic. For example, the semiconductor industry demands extremely tight purity specifications. As they are the biggest players/buyers in the game, the Si vendors do extensive testing and reporting on the purity of their silicon. To the contrary, nickel is heavily used in metal castings were purity is not as critical except for some specific contaminants - sulfur for example. Hence vendors will limit their assays to the elements of concern. Also, testing can be expensive so vendors will limit the assays to avoid paying for tests that are not required by the customer.

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What is the difference between atomic and weight based reporting?

Most reports use ppm by weight; however, some reports are in ppm atomic. The difference is shown in the following example:

Matrix Impurity Reported as Weight ppm Reported as Atomic ppm 
Tungsten Oxygen 175 ppm 2011 ppm  
Iron Oxygen 175 ppm 611 ppm  
Aluminum Oxygen 175 ppm 295 ppm  
Lithium Oxygen 175 ppm 76 ppm  

Reporting the impurity concentration in weight percent masks the true atom-to-atom ratio of the impurity. If we were only concerned with the oxygen in the tungsten, we could state that the tungsten is 99.98% pure with respect to oxygen on a weight basis, or the tungsten is 99.8 % pure with respect to oxygen on an atomic basis. One must recognize this effect in order to keep purity in perspective, especially when the atomic weights are very different: impurity Z << matrix Z.

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What is the measurement error? (3)

Each analytical measurement is an experiment attempting to arrive at the true value of the measured quantity. The error in the measurement is:

   Error = measured value - true value

As we don't know the true value we don't really know the error. What we do know is the magnitude of the uncertainty in our measured value is based on our understanding of the methods employed. Error/uncertainty is typically not reported in certificates of analysis. This (±) error range will vary for each analysis technique; for example, the sum of the three constituent elements in a ternary alloy may add up to 102% in an ICP measurement where the relative error is ± 3%.

It has become the norm that analytical assay numbers are reported without error ranges.  It is up to the individual to have an understanding that each method has its particular accuracy and precision.

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How do you achieve precise stoichiometry in alloying?

Generally alloys are provided with a nominal composition, that is, it is assumed what is put in the melt is what ends up in the alloy. For arc-casting of elements, alloy constituents are weighed to ±0.0005 gram accuracy, and then the cast weight is compared to the input masses.  The table below demonstrates a mass balance:

  Element Calculated Mass Actual Mass IN Percent Difference  
  Lanthanum 31.5521 g 31.5521  g -  
  Nickel 15.6200  g 15.6205  g 0.003%  
  Tin 2.9363  g 2.9362  g -0.003%  
    _____________ _____________    
  IN 50.1084  g 50.1008  g 0.001%  
  OUT   50.0917  g -0.034%  

We assume that the element is 100% pure in the calculations. This is the first error since there is no such thing as 100% purity. The second error is that the input masses are not equal to the calculated mass. In the example above the alloy lost 0.0091 grams of mass. Until further tests are made the effect of this loss is unknown. To compound the problem, when using an element with a high vapor pressure at its melting point, volatility becomes an issue.

The only way to achieve a high degree of confidence in compositional accuracy for bulk processing is to make a material, measure mass losses, analyze the material for some key factors (ICP, XRD, metallographically, etc. - see next section), make an educated adjustment to the mass balance if necessary, and then make the material again. This is why materials research can quickly become expensive.

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Will the true stoichiometry please stand up?

We may never know the true value of the elements in an alloy; however, one can use an array of other tools to conclude the stoichiometry is correct. XRD demonstrates the expected structure and lattice constants, physical property measurements are consistent with expectations (magnetism, heat capacity, and so on), and metallography demonstrates expected microstructures, etc. The degree to which the collected evidence is self-consistent will determine confidence in the stoichiometry of the alloy.

  Element Calculated Mass Actual Mass IN Percent Difference  
  Lanthanum 31.5521 g 31.5521  g -  
  Nickel 15.6200  g 15.6205  g 0.003%  
  Tin 2.9363  g 2.9362  g -0.003%  
    _____________ _____________    
  IN 50.1084  g 50.1008  g 0.001%  
  OUT   50.0917  g -0.034%  

The above casting example is from a hydrogen absorption alloy for the ESA Planck Mission.  Mass losses were specified to not exceed 0.1%; microprobe line scans were use to check homogeneity following annealing; XRD was used to check for the expected structure and lattice parameters, and finally isotherm data on H absorption was used to qualify the alloy performance for acceptance.

Wow, that is a lot of work! Yes, but if stoichiometry is critical the additional work is necessary.

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How do I convert atom percents to weight and volume percents?

Atomic%Weight% Converter. Download a simple MS Excel workbook which calculates conversions of atom percent formulas to weight and volume percents, and vise versa. Element data is inputted via a customizable look-up table.
At_Wt_Converter20080909.xls  (116 kb)†  Excel 2004 version 
At_Wt_Converter20080909.xlsx (  47 kb)† Excel 2008 version

Metallurgy Assistant is REALBasic® built package which performs atom percent formulas conversion to weight and volume percent, and vise versa, mass balances for alloy preparation, swaging area reduction calculations, ideal gas law pressure calculations, temperature conversions, and casting volume calculations (specific to MPC molds). The software also provides basic element data.

Metallurgy Assistant Version 3.0 (Sit 1.05 MB)†
    [Systems: Mac OS X 10.1, 10.2, untested on 10.3]

Metallurgy Assistant Version 3.2 (Dmg 5 MB) (Zip 1.2 MBDmg.Zip 1.2 MB) †
    [Systems: Mac OS X 10.4, untested on 10.3, 10.2,or 10.1]

† Commercial Liability and Disclaimer: Ames Laboratory, Iowa State University, U.S. DOE, and the Material Preparation Center make no claim to the accuracy of the calculated results. There is no expressed or implied warrantee.

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