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Frequently Asked Questions at the MPC

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FAQ for everyone

How long has the Materials Preparation Center been in operation?

While the MPC launched in its current form in 1982, Ames Laboratory has been actively involved in the preparation of very pure rare-earth metals since the early 1940s, before it was even a national laboratory. As part of the Manhattan project, a group of pioneering metallurgists at Iowa State College developed an ion-exchange method that was ultimately named after the college’s hometown (the Ames Process), and used to purify the large quantities of uranium needed to sustain the world’s first artificial nuclear reactor. Because of its contributions to the dawn of the Atomic Age and the defense of the nation during WWII, Ames became a National Laboratory in 1947, and it’s been pursuing advanced metals purification and alloying processes ever since. 

What are rare earth metals?

They are elements 56 through 71 on the Periodic Table of the Elements, plus scandium and yttrium: cerium (Ce), dysprosium (Dy), erbium (Er), europium (Eu), gadolinium (Gd), holmium (Ho), lanthanum (La), lutetium (Lu), neodymium (Nd), praseodymium (Pr), promethium (Pm), samarium (Sm), scandium (Sc), terbium (Tb), thulium (Tm), ytterbium (Yb), and yttrium (Y).

 

When alloyed with other metals, the rare-earths provide enhanced magnetic, strength and high-temperature and properties, and are important for applications in high-performance magnets, electronics, chemical catalysts, and clean energy technologies.

How pure are Ames Laboratory's rare earth metals?

Up to 99.99% pure. For sourcing research materials, purity matters. Without them, researchers may miss the true physical properties of the material, which results in missed opportunities to refine theories or develop applications.

FAQ for clients

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 impurities.

 

The table below explains 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.

What is the difference between atomic and weight based reporting?

Definitions of purity are often confusing, as there is a difference between absolute purity and the common metals basis purity.

 

Absolute purity, 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 table.

 

Metals basis purity, as is commonly indicated in catalogs, does not include many of the metallurgically important interstitial elements or other nonmetallic impurities.

 

For example, a certificate of analysis that declares Mn is 99.95% purity (min. metals basis) also reports 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. 

 

Some certificates of analysis 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? Because testing can be expensive and vendors are pragmatic. Assays are often limited to only those required by the customer. For example, the semiconductor industry demands extremely tight purity specifications, so silicon vendors do extensive testing and reporting on the purity of their silicon. On the other hand, nickel is heavily used in metal castings where purity is not as critical except for some specific contaminants. Hence vendors limit their assays to the elements of concern. 

 

For rare earth metals, purities are often reported on a TREM or Total Rare Earth Metal basis. For example, a report for Lanthanum may look like La 99.99% or La/TREM+ 99.99%. This is the purity of the Lanthanum relative to all other rare-earth metals only. The absolute purity with respect to all the other possible elements is not given by this calculation.

What is the measurement error?

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.

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.

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 used 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.

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

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